fix: adopt typeguard version 4.0

[alexander-held]

Relax the requirement of typeguard<3 introduced in #392 and adopt version 4.0.

The update of typeguard however now interacts with code coverage calculations, lowering the reported coverage: agronholm/typeguard#356. To avoid this, run pytest for Python 3.8 without typeguard to calculate coverage, and run typeguard only for the remaining Python versions.

resolves #391

* require typeguard>=4.0
* calculate code coverage using Python 3.8 without typeguard enabled
* remove typing workarounds that are no longer required

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (10525d0) 100.00% compared to head (ff2d1ff) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #406   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2068      2068           
  Branches       334       334           
=========================================
  Hits          2068      2068           

Impacted Files	Coverage Δ
src/cabinetry/smooth.py	100.00% <100.00%> (ø)

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