[pre-commit.ci] pre-commit autoupdate (#169)

updates:
- [github.com/psf/black: 21.5b2 → 21.6b0](psf/black@21.5b2...21.6b0)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

.pre-commit-config.yaml

@@ -12,7 +12,7 @@ repos:
         additional_dependencies: [toml]
         exclude: docs/pages
   - repo: https://github.com/psf/black
-    rev: "21.5b2"
+    rev: "21.6b0"
     hooks:
       - id: black
   - repo: https://github.com/kynan/nbstripout