Drop support for Python 3.7

[johannaengland]

This closes #2891 and is one step in fixing #2790. Can only be merged once we have released NAV 3.10.

The things Codecov complains about are fixed in #2903.

The things this PR does:

1. Update docker, tox, readthedocs, sonarcloud, Github workflows to not use Python 3.7 and only Python 3.9/3.10.
2. Upgrade dependencies that were stuck to an old version due to it being the last one supporting Python 3.7.
3. Update docs and pyproject.toml to mention minimum requirement of Python 3.9.

[github-actions]

Test results

       9 files         9 suites   7m 39s ⏱️
3 320 tests 3 320 ✔️ 0 💤 0 ❌
6 379 runs  6 379 ✔️ 0 💤 0 ❌

Results for commit 4088f09.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.19%. Comparing base (35f1a9b) to head (4088f09).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2901      +/-   ##
==========================================
- Coverage   60.20%   60.19%   -0.02%     
==========================================
  Files         601      601              
  Lines       43981    43981              
==========================================
- Hits        26480    26475       -5     
- Misses      17501    17506       +5     

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[lunkwill42]

This closes #2891 and is one step in fixing #2790. Can only be merged once we have released NAV 3.11.

I assume you mean NAV 5.10 ? 5.11 is the first release that will no longer support Pythons < 3.9.