Remove pytz dependency

[djhoese]

Python builtin datetime.timezone serves the same purpose

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.83%. Comparing base (c4d523a) to head (16f71f7).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   87.84%   87.83%   -0.01%     
==========================================
  Files          16       16              
  Lines        2632     2630       -2     
==========================================
- Hits         2312     2310       -2     
  Misses        320      320              

Flag	Coverage Δ
unittests	87.83% <100.00%> (-0.01%)	⬇️

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[coveralls]

coverage: 87.798% (-0.009%) from 87.807%
when pulling 16f71f7 on djhoese:remove-pytz-dep
into 03e220f on pytroll:main.

[pnuu]

I'm fine with the change, although I thought I already fixed all the datetime imports earlier on.