Ignore "unreachable" false-positives from mypy

[jwodder]

A bug in mypy's new unreachability-detection algorithm led to several false-positives when type-checking. This PR instructs mypy to just ignore what it thinks are errors.

[yarikoptic]

cool, thanks for the reference to the original issue! IMHO it would be useful to have that reference either in commit message or in a single use spot as a comment, so later we do not need to do much of investigation to figure out why those ignores and either it is time to remove them.

[jwodder]

@yarikoptic Reference added to commit message.

[codecov-commenter]

Codecov Report

Merging #142 (ba72e85) into master (23bcccc) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   70.97%   70.87%   -0.11%     
==========================================
  Files          10       10              
  Lines         975      975              
  Branches      179      179              
==========================================
- Hits          692      691       -1     
- Misses        226      230       +4     
+ Partials       57       54       -3     

Impacted Files	Coverage Δ
src/tinuous/github.py	51.56% <0.00%> (-1.35%)	⬇️
src/tinuous/__main__.py	53.47% <0.00%> (+1.38%)	⬆️

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[yarikoptic]

Thank you!