fix: update requires-python to include Python 3.14

[yurivski]

Fixes #1811

The pyproject.toml lists Programming Language :: Python :: 3.14 in classifiers
but requires-python was set to >=3.10,<3.14, preventing installation on Python 3.14.

Changes

• Updated requires-python from <3.14 to <3.15

Testing

• Full test suite passed on Python 3.13.5 (2195 passed, 4 skipped, 0 failures)
• Spark backend tests excluded (optional dependency)

[akx]

Full test suite passed on Python 3.13.5 (2195 passed, 4 skipped, 0 failures)

Did you try installing or testing on Python 3.14?

[yurivski]

Did you try installing or testing on Python 3.14?

I tested on Python 3.13.5 — I don't have 3.14 installed locally.
The test suite passed fully (2195 passed, 4 skipped, 0 failures).

The main motivation for this change is that the classifiers already list
Python 3.14 support, but requires-python was blocking installation.
Since the htmlmin dependency (which previously blocked newer Python versions)
was removed in 4.17.0, the upper bound seemed outdated.

Happy to run additional tests if needed.