Juno: mouseover_set attribute was deprecated in Matplotlib 3.0

Dear all,

I have encountered a strange error message thrown by Atom (Juno) on macOS.
Not sure if this is the right place because in the REPL this error is not present.

Steps to reproduce:
- install Juno package for Atom
- install PyPlot.jl and link it to already existing Anaconda installation

within Juno first execute this bit of code:
```julia
using PyPlot
fig, ax = PyPlot.subplots()
ax.plot(rand(10))
PyPlot.close()
```
and then:
as soon as you type `ax.` the following error message is thrown within Atom's error message:

**Julia Client – Internal Error:**
```
LoadError: PyError ($(Expr(:escape, :(ccall(#= ~/.julia/packages/PyCall/zqDXB/src/conversions.jl:96 =# @pysym(PyString_AsStringAndSize), Cint, (PyPtr, Ptr{Ptr{UInt8}}, Ptr{Int}), po, _ps_ptr, _ps_len)))))
TypeError('expected bytes, NoneType found')
in expression starting at string:1
pyerr_check at exception.jl:60 [inlined]
pyerr_check at exception.jl:64 [inlined]
_handle_error(::String) at exception.jl:81
macro expansion at exception.jl:95 [inlined]
convert(::Type{String}, ::PyCall.PyObject) at conversions.jl:96
getdoc at PyCall.jl:903 [inlined]
getdoc(::Any, ::Any) at Docs.jl:254
doc(::Base.Docs.Binding, ::Type) at docview.jl:86
doc(::Base.Docs.Binding) at docview.jl:85
top-level scope at string:1
include_string(::Module, ::String, ::String) at loading.jl:1075
```

and the REPL prints out this error message:
```
~/anaconda3/lib/python3.7/inspect.py:341: MatplotlibDeprecationWarning:
The mouseover_set attribute was deprecated in Matplotlib 3.0 and will be removed in 3.2.
```

报告指出应检查 inspect.py:341，以及通过 PyCall 的 conversions.jl:96、PyCall.jl:903 和 Docs.jl:254 的 Julia 堆栈。使用 PyPlot 后复现 Juno/Atom 流程，然后跟踪弃用警告是否导致 NoneType 转换；当输入 ax. 不再触发内部错误时即表示完成。

由索引模型根据 Issue 内容生成。