Add type-checking using mypy

[ubdbra001]

closes #43

Adds type-checking using mypy to the CI with tox.

[codecov]

Codecov Report

❌ Patch coverage is 89.89899% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.56%. Comparing base (92c6852) to head (c3dcb23).

Files with missing lines	Patch %	Lines
src/bids_validator/context.py	60.00%	7 Missing and 1 partial ⚠️
src/bids_validator/types/_typings.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   90.95%   90.56%   -0.40%     
==========================================
  Files          12       13       +1     
  Lines         774      795      +21     
  Branches      116      120       +4     
==========================================
+ Hits          704      720      +16     
- Misses         46       48       +2     
- Partials       24       27       +3     

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[ubdbra001]

@effigies Here's an initial attempt at adding type-checking using mypy to the CI, though I don't think this is how you envisaged it working. What do you think?

[effigies]

Yup, this looks good to me. Now to start fixing the errors... I may push a few things.

[effigies]

Reenabling strict mode and some initial fixes...