Bump urllib3 from 1.26.3 to 1.26.4

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.3 to 1.26.4.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/1.26.4/CHANGES.rst)
- [Commits](urllib3/urllib3@1.26.3...1.26.4)

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

Author: dependabot-preview[bot]

docs-requirements.txt

@@ -81,5 +81,5 @@ toml==0.10.2
     # via towncrier
 towncrier==19.2.0
     # via -r docs-requirements.in
-urllib3==1.26.3
+urllib3==1.26.4
     # via requests