Bring back arbitrary **config to Python handler

[florimondmanca]

Closes #154

I'm not sure if this is what we want, or if we want to not pass **config in the parent get_handler() implementation, but either of these should resolve the bug showcased in #154.

https://github.com/florimondmanca/mkdocstrings/blob/25373148699daad42b319fa04213d405ca8a7c4c/src/mkdocstrings/handlers/__init__.py#L244

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PS: I wasn't able to run make setup (I don't have 3.6 and 3.7 installed, and pinning PYTHON_VERSIONS = ("3.8",) results in a strange error I couldn't find the cause of…

$ make setup
Setting up Python 3.8 environment
---------------------------------
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'PosixPath' is not defined
make: *** [setup] Error 1

[pawamoy]

Hello! Yeah I naively removed the **kwargs because they weren't used in the function, and it didn't fail on my side because I don't actually use any config on my autodoc instructions 😩

This definitely motivates me to finally write more tests 😅

[pawamoy]

About make setup, it's just a way to install environments for multiple Python version, but you can simply run poetry install instead (the things that run on multiple interpreters should be skipped then). I'll add this in the contributing docs, thanks 🙂