Implement support for the free-threaded build of CPython 3.13

[lysnikolaou]

What do these changes do?

• Change Cython dependency to 3.1.0a1 for CPython 3.13 to support free-threading
• Test the free-threaded build on CI
• IMPORTANT: There's still some thread-safety issues regarding the frozen attribute of FrozenList, which I would like to wait for Add critical_section decorator cython/cython#6577 for. In practice that won't be hit too often, so I'm opening this PR already.

Are there changes in behavior for the user?

No.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modifications, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep the list in alphabetical order, the file is sorted by name.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."