Add typehints and code improvements across the board

[jonded94]

• Removed file descriptor leak in tests
• Added typehints across the library
• Almost mypy --strict compatible; would need drop of Python 3.8 compatibility to reach that; decided against that
• Removed unreachable code
• Added support for bool as option argument
• Small spelling correction
• Added py.typed marker file for mypy

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.63014% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.63%. Comparing base (84a5fc4) to head (6fb23a1).
Report is 5 commits behind head on master.

Files	Patch %	Lines
tidy/test_tidy.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   98.14%   98.63%   +0.48%     
==========================================
  Files           4        4              
  Lines         270      292      +22     
==========================================
+ Hits          265      288      +23     
+ Misses          5        4       -1     

Flag	Coverage Δ
unittests	98.63% <98.63%> (+0.48%)	⬆️

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[nijel]

Merged, thanks for your contribution!