chore: Update pre-commit hook revs

[matthewfeickert]

* Update Black pre-commit hook to use rev v20.8b1
   - Using pinned revs is the recommended approach for pre-commit: https://github.com/psf/black/issues/420
* Update flake8 pre-commit hook to use rev v3.8.3

[codecov]

Codecov Report

Merging #40 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   31.67%   31.67%           
=======================================
  Files           1        1           
  Lines         161      161           
  Branches       35       35           
=======================================
  Hits           51       51           
  Misses        108      108           
  Partials        2        2           

Flag	Coverage Δ
#unittests	31.67% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e4eeaaa...92c4131. Read the comment docs.