start testing min required dependencies

[mathause]

• Towards add minimum supported versions #175 (but not closing it)
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

Start testing the lowest version of our supported dependencies. We don't specify the all of them as of yet. But with #396 and #397 this becomes relevant.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.95%. Comparing base (9b0b76b) to head (3b31f76).
Report is 7 commits behind head on main.

❗ Current head 3b31f76 differs from pull request most recent head b30d795. Consider uploading reports for the commit b30d795 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #398      +/-   ##
==========================================
+ Coverage   87.90%   87.95%   +0.04%     
==========================================
  Files          40       40              
  Lines        1745     1743       -2     
==========================================
- Hits         1534     1533       -1     
+ Misses        211      210       -1     

Flag	Coverage Δ
unittests	87.95% <ø> (+0.04%)	⬆️

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[mathause]

Actually version 0.9 of regionmask was required even before (for the new code path)...

[mathause]

Gentle ping @veni-vidi-vici-dormivi