hightime: Add support for Python 3.10-3.13 and drop Python 3.7-3.8

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Update Python versions in tox.ini and trove classifiers to match NI's Python support policy.

Update datetime to use __index__ to implicitly convert parameters to int, rather than using __int__. This follows python/cpython#15636 from Python 3.10.

Why should this Pull Request be merged?

Fixes #42

The GitHub-hosted runner no longer supports Python 3.7 or 3.8.

What testing has been done?

Ran pyenv global 3.9.13 3.10.11 3.11.9 3.12.9 3.13.2 and tox:

  py39-test: OK (1.44=setup[0.08]+cmd[0.05,0.09,1.22] seconds)
  py310-test: OK (1.44=setup[0.01]+cmd[0.05,0.11,1.26] seconds)
  py311-test: OK (1.44=setup[0.00]+cmd[0.06,0.08,1.30] seconds)
  py312-test: OK (1.55=setup[0.00]+cmd[0.06,0.09,1.39] seconds)
  py313-test: OK (1.51=setup[0.01]+cmd[0.03,0.08,1.39] seconds)
  pypy-test: SKIP (0.03 seconds)
  pypy3-test: SKIP (0.03 seconds)
  flake8: OK (0.83=setup[0.01]+cmd[0.06,0.08,0.67] seconds)
  black: OK (0.80=setup[0.00]+cmd[0.05,0.09,0.66] seconds)
  congratulations :) (9.17 seconds)

Verified that pytest prints "Python 3.13.2" etc.