Bump notebook from 6.3.0 to 6.4.1

Bumps [notebook](http://jupyter.org) from 6.3.0 to 6.4.1.

---
updated-dependencies:
- dependency-name: notebook
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements-dev.txt

@@ -1,4 +1,4 @@
 coverage>=4.5.3
 pytest-cov>=2.7.1
-notebook==6.3.0
+notebook==6.4.1
 jupyter==1.0.0