Remove workaround for mypy not paying attention to type() comparisons

[altendky]

Stalled on:

• "if type(x) == T" not being used for type narrowing python/mypy#4445
• release of ^^^

Draft for:

• Updating mypy requirement to the new version

[codecov]

Codecov Report

Merging #141 (1cf523a) into main (4c62f78) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #141   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         2015      2015           
  Branches       138       140    +2     
=========================================
  Hits          2015      2015           

Impacted Files	Coverage Δ
qtrio/_core.py	100.00% <ø> (ø)
qtrio/_exceptions.py	100.00% <ø> (ø)

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[altendky]

Oh well, not working. Dig into it at some point...