drop python 3.7 support

[mathause]

• Closes drop support for python 3.7 #163
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f5d3e5) 87.95% compared to head (f5884fb) 87.72%.

❗ Current head f5884fb differs from pull request most recent head b6808fb. Consider uploading reports for the commit b6808fb to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   87.95%   87.72%   -0.23%     
==========================================
  Files          40       40              
  Lines        1752     1752              
==========================================
- Hits         1541     1537       -4     
- Misses        211      215       +4     

Flag	Coverage Δ
unittests	87.72% <ø> (-0.23%)	⬇️

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[mathause]

python 3.7 is EOL so this was overdue: https://devguide.python.org/versions/