Add postcss as a dependency

[emmatown]

What:

Why:

It fixes some weirdness since we import things from postcss but it's not one of our explicit dependencies.

How:

Checklist:

• Documentation
• Tests
• Code complete

[codecov-io]

Codecov Report

Merging #233 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #233   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files          20       20           
  Lines         971      971           
  Branches      263      263           
=======================================
  Hits          877      877           
  Misses         76       76           
  Partials       18       18

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