[pre-commit.ci] pre-commit autoupdate (#161)

updates:
- [github.com/psf/black: 20.8b1 → 21.4b0](psf/black@20.8b1...21.4b0)
- [github.com/kynan/nbstripout: 0.3.9 → 0.4.0](kynan/nbstripout@0.3.9...0.4.0)

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

.pre-commit-config.yaml

@@ -12,10 +12,10 @@ repos:
         additional_dependencies: [toml]
         exclude: docs/pages
   - repo: https://github.com/psf/black
-    rev: "20.8b1"
+    rev: "21.4b0"
     hooks:
       - id: black
   - repo: https://github.com/kynan/nbstripout
-    rev: "0.3.9"
+    rev: "0.4.0"
     hooks:
       - id: nbstripout