Add Python 3.13 and 3.14; drop 3.8

[twm]

• Add Python 3.13 and 3.14 to the test matrix
• Update pinned dependencies as necessary to get the build to pass
• Remove Python 3.8 because test dependencies have dropped it

There are no code changes that would prevent Incremental from working on Python 3.8, so I haven't adjusted the requires_python, but it's no longer considered supported.

[adiroiban]

Looks good. Many thanks for the update!

[twm]

Thank you for the review @adiroiban!