Fix some issues with mypy

[drhagen]

This was a failed attempt at #37. This merges are some fixes that mypy recommended. There is too much magic in Parsita to fix them all, so mypy is not made a dependency of Parsita at this time.

[codecov-commenter]

Codecov Report

Merging #61 (a5c53bd) into master (de91b9a) will not change coverage.
The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##            master       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines          830       830           
  Branches        99        99           
=========================================
  Hits           830       830           

Impacted Files	Coverage Δ
src/parsita/state/_reader.py	100.00% <ø> (ø)
src/parsita/parsers/_alternative.py	100.00% <100.00%> (ø)
src/parsita/parsers/_any.py	100.00% <100.00%> (ø)
src/parsita/parsers/_base.py	100.00% <100.00%> (ø)
src/parsita/parsers/_conversion.py	100.00% <100.00%> (ø)
src/parsita/parsers/_debug.py	100.00% <100.00%> (ø)
src/parsita/parsers/_end_of_source.py	100.00% <100.00%> (ø)
src/parsita/parsers/_literal.py	100.00% <100.00%> (ø)
src/parsita/parsers/_optional.py	100.00% <100.00%> (ø)
src/parsita/parsers/_predicate.py	100.00% <100.00%> (ø)
... and 8 more

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