Drop support for EOL Python 3.7

[hugovk]

Changes proposed in this pull request:

• Drop support for Python 3.7, EOL this month
  • https://devguide.python.org/versions/
  • https://peps.python.org/pep-0537/
• Not planning a major bump, because requires-python = ">=3.8" will make sure people get the right version
• Fix a test that checking for .gif, the data is now .png
• Some test refactoring, including replace helper methods that were useful with unittest and less so with pytest

[codecov]

Codecov Report

Merging #430 (47eda3e) into main (8d8263c) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #430      +/-   ##
==========================================
- Coverage   90.07%   90.02%   -0.06%     
==========================================
  Files          12       12              
  Lines        2186     2185       -1     
==========================================
- Hits         1969     1967       -2     
- Misses        217      218       +1     

Flag	Coverage Δ
ubuntu-latest	90.02% <100.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pylast/__init__.py	89.70% <100.00%> (-0.11%)	⬇️
tests/test_album.py	100.00% <100.00%> (ø)
tests/test_library.py	100.00% <100.00%> (ø)
tests/test_librefm.py	100.00% <100.00%> (ø)
tests/test_network.py	89.63% <100.00%> (ø)
tests/test_pylast.py	100.00% <100.00%> (ø)
tests/test_user.py	100.00% <100.00%> (ø)