Add Py_mod_gil slot to C extension modules
x-ref #59057.

The `Py_mod_gil` slot is needed so that the GIL is not automatically enabled when these C extension modules get imported.

- I checked `pandas_datetime` for thread-safety and it appears to be okay.
- `pandas_parser` is not thread-safe. A discussion is needed on whether we should care about it though, since the `c` engine to `read_csv` and friends is already documented as being thread-unsafe and the thread-safe `pyarrow` alternative is already there.
Is this slot only available in 3.13?

> * `pandas_parser` is not thread-safe. A discussion is needed on whether we should care about it though,

Do you know what makes it not thread unsafe? I'm surprised given how small of a module it is.

Definitely more of a nice-to-have on the parser though. Understood its probably a very large undertaking to make the CSV reader overall multi-threaded, and most likely not even worth it with pyarrow > Do you know what makes it not thread unsafe? I'm surprised given how small of a module it is.

I mean that it's thread-unsafe in the sense that there's global state in the `parser_t` struct and there's unsafe reads/writes to it. As far as I understand, `read_csv` only ever creates one instance of it though, which means that running `read_csv` in multiple threads is safe.  Sorry for the typo - I mean "not thread safe" but I think you got what I was saying. And cool thanks for the response - agree with you on that Thanks @lysnikolaou 