Convenient syntax for `setproperty!`

I couldn't find an earlier issue but this seems like something that would be a common issue, so please share if there's a related one.

Basically, I think the following code should work:

```python
from julia import Main as jl

jl.eval("""
mutable struct MyStruct
x::Int
end
""")

obj = jl.MyStruct(1)
obj.x = 2
```

However, it gives the following error:

```python
Traceback (most recent call last):
File "/Users/mcranmer/Documents/ManimAnimation/SR/setting_demo.py", line 10, in
obj.x = 2
AttributeError: 'PyCall.jlwrap' object has no attribute 'x'
```

I couldn't completely figure out where `__setattr__` should be overloaded, but I feel like it would be possible to enable this. For the record, `__getattr__` seems to work fine; it's just `__setattr__` that doesn't work.

As a temporary workaround, I am using the function:

```python
from julia import Main as jl
jl.eval("""
import Base: setproperty!
setproperty!(obj, field::String, val) = setproperty!(obj, Symbol(field), val)
""")

set = jl.setproperty_b

jl.eval("""
mutable struct MyStruct
x::Int
end
""")

obj = jl.MyStruct(1)
set(obj, "x", 2)

print(obj.x)
# 2
```
But I think `obj.x = 2` should do this automatically.

(needed to overload `setproperty!` - see https://github.com/JuliaPy/PyCall.jl/issues/11)

---

cc @mkitti @marius311 @stevengj