TST: Test Pyside6 and Python 3.13

[larsoner]

I was going to open a PR to conda-forge to add Python 3.13 packages, but realized I should probably create a PR here for 3.13 support here. From a very quick search I didn't find an issue about it (#22801 was close I guess?), but I did find #20201. So this PR tries to take care of both by adding a CI job for PySide6 and Python 3.13. Then also updates setup.py, and tweaks the README to promote the newer PySide6.

[larsoner]

There are segfaults that look like:

Current thread 0x00007f70d8fa5440 (most recent call first):
  File "/home/runner/work/spyder/spyder/spyder/plugins/preferences/tests/conftest.py", line 31 in <module>
  File "/home/runner/micromamba/envs/test/lib/python3.13/site-packages/_pytest/assertion/rewrite.py", line 186 in exec_module

which seems to have to do with

spyder/spyder/plugins/preferences/tests/conftest.py

Lines 31 to 32 in 0060d83

	class MainWindowMock(QMainWindow):
	register_shortcut = Mock()

Specifically if I remove the class attribute then it at least won't segfault, but then there are errors later. So I'm not sure what the correct fix is 🙁