ci(mypy): Turn on mypy --strict

[tony]

See also:

• https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-strict
• Strict mypy libvcs#390
• Fixes Strict mypy annotations #385

[codecov]

Codecov Report

Merging #386 (e8f81df) into master (f7f4d96) will decrease coverage by 1.36%.
The diff coverage is 78.10%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
- Coverage   85.37%   84.00%   -1.37%     
==========================================
  Files          16       17       +1     
  Lines         889      938      +49     
  Branches      157      168      +11     
==========================================
+ Hits          759      788      +29     
- Misses         89      100      +11     
- Partials       41       50       +9     

Impacted Files	Coverage Δ
tests/test_cli.py	88.23% <ø> (ø)
src/vcspull/validator.py	40.00% <40.00%> (ø)
src/vcspull/log.py	68.42% <46.15%> (ø)
src/vcspull/cli/sync.py	72.09% <61.90%> (-1.25%)	⬇️
conftest.py	78.04% <83.33%> (ø)
tests/test_config.py	96.96% <90.90%> (-3.04%)	⬇️
tests/test_sync.py	90.27% <92.30%> (-1.03%)	⬇️
src/vcspull/_internal/config_reader.py	94.44% <100.00%> (ø)
src/vcspull/config.py	84.67% <100.00%> (-1.36%)	⬇️
src/vcspull/types.py	100.00% <100.00%> (ø)
... and 7 more

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