PyCall + Distributed: cannot pickle 'traceback' object

Hi, I posted on [Discourse](https://discourse.julialang.org/t/distributed-pycall-cannot-see-exceptions-cannot-pickle-traceback-object/59802) but no answer so far, so maybe it's better here?
I have trouble getting the error type/message when running PyCall with `@distributed` (when exceptions are raised in Python). It seems that it's the stack trace which poses problem, but I don't know if it is a known limitation or a bug? From PyCall or from Distributed?

```julia
julia> using Distributed

julia> addprocs(1)
1-element Vector{Int64}:
2

julia> @everywhere using PyCall

julia> fetch(@spawnat 2 py"""
raise ZeroDivisionError
""")
┌ Error: Fatal error on process 2
│ exception =
│ PyError ($(Expr(:escape, :(ccall(#= /home/me/.julia/packages/PyCall/BD546/src/pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw)))))
│ TypeError("cannot pickle 'traceback' object")
│
│ Stacktrace:
│ [1] pyerr_check
│ @ ~/.julia/packages/PyCall/BD546/src/exception.jl:62 [inlined]
│ [2] pyerr_check
│ @ ~/.julia/packages/PyCall/BD546/src/exception.jl:66 [inlined]
│ [3] _handle_error(msg::String)
│ @ PyCall ~/.julia/packages/PyCall/BD546/src/exception.jl:83
│ [4] macro expansion
│ @ ~/.julia/packages/PyCall/BD546/src/exception.jl:97 [inlined]
│ [5] #107
│ @ ~/.julia/packages/PyCall/BD546/src/pyfncall.jl:43 [inlined]
│ [6] disable_sigint
│ @ ./c.jl:458 [inlined]
│ [7] __pycall!
│ @ ~/.julia/packages/PyCall/BD546/src/pyfncall.jl:42 [inlined]
│ [8] _pycall!(ret::PyObject, o::PyObject, args::Tuple{PyObject}, nargs::Int64, kw::Ptr{Nothing})
│ @ PyCall ~/.julia/packages/PyCall/BD546/src/pyfncall.jl:29
│ [9] _pycall!
│ @ ~/.julia/packages/PyCall/BD546/src/pyfncall.jl:11 [inlined]
│ [10] #pycall#112
│ @ ~/.julia/packages/PyCall/BD546/src/pyfncall.jl:80 [inlined]
│ [11] pycall
│ @ ~/.julia/packages/PyCall/BD546/src/pyfncall.jl:80 [inlined]
│ [12] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, pyo::PyObject)
│ @ PyCall ~/.julia/packages/PyCall/BD546/src/serialize.jl:14
│ [13] serialize_any(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)
│ @ Serialization /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Serialization/src/Serialization.jl:649
│ [14] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)
│ @ Serialization /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Serialization/src/Serialization.jl:628
│ [15] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, ex::CapturedException)
│ @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/clusterserialize.jl:205
│ [16] serialize_any(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)
│ @ Serialization /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Serialization/src/Serialization.jl:649
│ [17] serialize(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, x::Any)
│ @ Serialization /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Serialization/src/Serialization.jl:628
│ [18] serialize_msg(s::Distributed.ClusterSerializer{Sockets.TCPSocket}, o::Distributed.ResultMsg)
│ @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/messages.jl:78
│ [19] #invokelatest#2
│ @ ./essentials.jl:708 [inlined]
│ [20] invokelatest
│ @ ./essentials.jl:706 [inlined]
│ [21] send_msg_(w::Distributed.Worker, header::Distributed.MsgHeader, msg::Distributed.ResultMsg, now::Bool)
│ @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/messages.jl:174
│ [22] send_msg_now
│ @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/messages.jl:118 [inlined]
│ [23] send_msg_now(s::Sockets.TCPSocket, header::Distributed.MsgHeader, msg::Distributed.ResultMsg)
│ @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/messages.jl:113
│ [24] deliver_result(sock::Sockets.TCPSocket, msg::Symbol, oid::Distributed.RRID, value::RemoteException)
│ @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:95
│ [25] macro expansion
│ @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:286 [inlined]
│ [26] (::Distributed.var"#105#107"{Distributed.CallMsg{:call_fetch}, Distributed.MsgHeader, Sockets.TCPSocket})()
│ @ Distributed ./task.jl:406
└ @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:99
Worker 2 terminated.ERROR:
ProcessExitedException(2)
Stacktrace:
[1] try_yieldto(undo::typeof(Base.ensure_rescheduled))
@ Base ./task.jl:705
[2] wait
@ ./task.jl:764 [inlined]
[3] wait(c::Base.GenericCondition{ReentrantLock})
@ Base ./condition.jl:106
[4] take_buffered(c::Channel{Any})
@ Base ./channels.jl:389
[5] take!(c::Channel{Any})
@ Base ./channels.jl:383
[6] take!(::Distributed.RemoteValue)
@ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:599
[7] #remotecall_fetch#143
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:390 [inlined]
[8] remotecall_fetch(f::Function, w::Distributed.Worker, args::Distributed.RRID)
@ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:386
[9] #remotecall_fetch#146
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:421 [inlined]
[10] remotecall_fetch
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:421 [inlined]
[11] call_on_owner
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:494 [inlined]
[12] fetch(r::Future)
@ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/remotecall.jl:533
[13] top-level scope
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/macros.jl:99
```

Tái hiện lỗi với Distributed, addprocs(1), @everywhere bằng PyCall và ví dụ @spawnat. Trước tiên, hãy đọc src/serialize.jl và exception.jl của PyCall cùng với các frame serialization của Distributed được hiển thị trong báo cáo. Hoàn tất khi có thể lấy được kiểu và thông báo của exception Python từ xa mà không làm worker kết thúc.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.