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