pre-commit: autoupdate hook versions (#98)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

.pre-commit-config.yaml

@@ -14,7 +14,7 @@ repos:
       - id: isort
   # https://github.com/python/black#version-control-integration
   - repo: https://github.com/python/black
-    rev: 20.8b1
+    rev: 21.4b2
     hooks:
       - id: black
   - repo: https://github.com/keewis/blackdoc