[mod] pre-commit black revision stick to v21.5b0

https://pre-commit.com/#pre-commit-configyaml---repos

.pre-commit-config.yaml

 repos:
 -   repo: https://github.com/psf/black
-    rev: stable
+    rev: 21.5b0
     hooks:
     - id: black
 -   repo: https://github.com/PyCQA/isort