docwriter: Fix (out) arguments for Python

Niels De Graef requested to merge wip/nielsdg/python-formatter-out-params into master

The documentation formatter for Python3 doesn't properly deal with (out) arguments, which leads to those being added as function arguments, rather than return values. This commit fixes that by mostly copying the logic from the GJS formatter.

For future work, it might be worth looking into whether Python type annotations can be used to add types to arguments.

Fixes #235

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