Refactor remove legacy code support

[pnuu]

There were still some remainders of Python 2.7 support. This PR removes them.

• Closes Remove remnants of Python 2 support #62
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff

[codecov]

Codecov Report

Merging #67 (2c9854f) into main (8d1fe8f) will increase coverage by 0.11%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   46.49%   46.61%   +0.11%     
==========================================
  Files          15       15              
  Lines        2540     2525      -15     
==========================================
- Hits         1181     1177       -4     
+ Misses       1359     1348      -11     

Impacted Files	Coverage Δ
trollsched/satpass.py	63.22% <50.00%> (+0.40%)	⬆️
trollsched/schedule.py	15.16% <50.00%> (+0.07%)	⬆️
trollsched/tests/test_schedule.py	96.73% <100.00%> (+1.50%)	⬆️
trollsched/utils.py	14.81% <100.00%> (-0.67%)	⬇️

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