Add support for Python 3.15 and drop EOL 3.6-3.9

[hugovk]

The Python 3.15 release candidate is out. 🚀

The 3.15 release manager (👋 hi, that's me!) issued a call to action:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.15 during this phase, and publish Python 3.15 wheels on PyPI to be ready for the final release of 3.15.0, and to help other projects do their own testing. Any binary wheels built against Python 3.15.0 release candidates will work with future versions of Python 3.15. As always, report any issues to the Python bug tracker.

Please keep in mind that this is a preview release and while it's as close to the final release as we can get it, its use is not recommended for production environments.

Also drop little-used EOL Python 3.6-3.9:

https://devguide.python.org/versions/

category	percent	downloads
3.12	33.72%	176,832
3.11	31.32%	164,244
3.14	14.00%	73,421
3.13	9.96%	52,211
3.10	5.44%	28,532
3.9	2.77%	14,513
null	1.64%	8,577
3.8	0.51%	2,669
3.7	0.41%	2,146
2.7	0.14%	709
3.5	0.04%	221
3.6	0.04%	201
3.15	0.02%	109
Total		524,385

Source: pip install -U pypistats && pypistats python_minor gpxpy --last-month

[hugovk]

pyupgrade removed this, and I intentionally left it out: modern pip and uv respect python_requires so this sort of guard isn't needed any more.

[tkrajina]

Makes sense, I'll definitely merge it before I release the new version, but I'll just try to cleanup the other (older) issues and pull requests.