Python 3.13 support

[pllim]

When Python 3.13 is out, need to:

1. Add CI for py313 -- Done in Python 3.13 support #394
2. Add wheel build for py313 -- Done in Python 3.13 support #394
3. New release so the wheels get pushed out and for conda-forge to pick up (nightly already available)
4. Update CI to use py313 regularly

Was reminded by #392

[lpsinger]

You don't necessarily need to wait until Python 3.13 is released. There is a prerelease mode for actions/setup-python that ought to allow you to test against Python 3.13. Many packages (numpy, scipy) have already released wheels for Python 3.13.

[pllim]

Testing is done in #394 . Would be nice if I have a way to upload nightly wheel for synphot instead of doing a PyPI release before Python 3.13 stable is out. We will see...

Thanks for the ping!

[pllim]

BTW, I unleashed the nightly wheel at https://anaconda.org/STScI/synphot