chore: Update pre-commit hook revs (#40)

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

.pre-commit-config.yaml

@@ -1,10 +1,10 @@
 repos:
 -   repo: https://github.com/psf/black
-    rev: stable
+    rev: 20.8b1
     hooks:
     - id: black
       language_version: python3
 -   repo: https://gitlab.com/pycqa/flake8
-    rev: 3.8.1
+    rev: 3.8.3
     hooks:
     - id: flake8