🔧 Use the tag version for black in pre-commit

The recent revisions of the pre-commit integration with black has specified to use the version instead of a constant branch name like `stable` so that it could be updated seemlessly using `pre-commit autoupdate`. For reference,

- https://pre-commit.com/#using-the-latest-version-for-a-repository
- pre-commit/pre-commit#1354
- https://github.com/psf/black#version-control-integration
- psf/black#420

.pre-commit-config.yaml

@@ -1,6 +1,6 @@
 repos:
     - repo: https://github.com/python/black
-      rev: stable
+      rev: 20.8b1
       hooks:
       - id: black
         language_version: python3.6