Deprecate scipy Legendre function

[f-brinkmann]

Changes proposed in this pull request:

Note the following: I think the tests are failing due to an error in scipy scipy/scipy#23101

• Use scipy's new accoc_legendre instead of lpmn which will be deprecated in scipy 1.17
• Restrict scipy to version >= 1.15.0 is done in Deprecate scipy Spherical Harmonics function #142
• Add tests for spharpy.special.legendre_function to make sure changes work as intended
• Minor corrections in the docstrings

To Do

• Wait for bugfix in scipy 1.17 and update pyproject.toml accordingly, or fix bug internally as suggested in the issue linked above.

[ahms5]

Thank you for taking care. the failing test is really annoying^^
I'm note sure about every detail, otherwise seems to be fine

[ahms5]

🍻

[ahms5]

If it requires a certain scipy version, we should add it to the pyproject.toml

[f-brinkmann]

If it requires a certain scipy version, we should add it to the pyproject.toml

It scheduled for scipy 1.17, but there hasn't been much action on the issue. I added a todo to the top so we don't forget.

[mberz]

I'm not expecting that this will be solved in time for the next release of spharpy.
I think we either have to fix this ourselves or live with the bug.

[mberz]

There's an update on the respective scipy issue, let's wait for further updates here scipy/scipy#23101

[mberz]

I've created a PR to fix the underlying issue with the Legendre functions in scipy.
In any case, the issue is scheduled to be fixed no later than scipy 1.17.0, which is when scipy.special.lpmv is to be removed.
So it's probably a good idea to wait with this PR.