Silence warnings from Python files

The `Python.h` header must be included first to appease the preprocessor.
Additionally, it includes ``, ``, ``, ``,
`` and ``, so we don't have to.

Also:

* Mark Python code as `ssize_t` clean
We don't use any `#` format variants, but it's best to define this for any future usage.

* Remove unused struct members from Python loader

* Simplify `gplugin_python3_filename_to_module`
I believe the call to `g_utf8_strrchr` was in error, as the second argument is
length in bytes, but `g_utf8_strlen` returns length in characters, which is
shorter. In any case, it accepts -1 anyway, so there's no need to count the
length.

Testing Done:
Compile and `ninja test`

Reviewed at https://reviews.imfreedom.org/r/960/