Require Python >= 3.8

[takluyver]

This is still fairly conservative, but we're still using 3.8 for offline calibration at present, so I don't want to make extra work for ourselves if updating that gets held up.

[takluyver]

For the 3.11 tests, pytest crashes basically as soon as it starts. :confused:

   python3 -m pytest -v --nbval-lax --current-env --cov=extra_data --cov-report=xml
  shell: /usr/bin/bash -e {0}
  env:
    pythonLocation: /opt/hostedtoolcache/Python/3.11.9/x64
    PKG_CONFIG_PATH: /opt/hostedtoolcache/Python/3.11.9/x64/lib/pkgconfig
    Python_ROOT_DIR: /opt/hostedtoolcache/Python/3.11.9/x64
    Python2_ROOT_DIR: /opt/hostedtoolcache/Python/3.11.9/x64
    Python3_ROOT_DIR: /opt/hostedtoolcache/Python/3.11.9/x64
    LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.11.9/x64/lib
    MPLBACKEND: agg
Error: Process completed with exit code 1.

[takluyver]

Aha: https://github.com/pytest-dev/pytest-cov/issues/564

It seems like it needs coverage 6.3, which got lost in #272.

[takluyver]

I've updated coverage to the last 6.x version so the CI works. I'll let Dependabot try the update to 7.x separately afterwards.

[philsmt]

Thanks, LGTM!