Add a --pyproject option to towncrier.check

[exarkun]

Allows for a work-around for #123

[codecov-io]

Codecov Report

Merging #127 into master will increase coverage by 0.09%.
The diff coverage is 95%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   93.25%   93.35%   +0.09%     
==========================================
  Files          15       15              
  Lines         623      632       +9     
  Branches       61       64       +3     
==========================================
+ Hits          581      590       +9     
  Misses         26       26              
  Partials       16       16

Impacted Files	Coverage Δ
src/towncrier/check.py	93.33% <100%> (+0.47%)	⬆️
src/towncrier/test/test_check.py	100% <100%> (ø)	⬆️
src/towncrier/_settings.py	76.47% <80%> (+0.71%)	⬆️

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[hawkowl]

thanks @exarkun

[dstansby]

Would it be possible to add this as a command line option too?