Bump urllib3 from 1.26.5 to 1.26.18 in /doc/sphinx (#67)

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.5 to 1.26.18.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.26.5...1.26.18)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

doc/sphinx/requirements.txt

@@ -36,5 +36,5 @@ sphinxcontrib-htmlhelp==1.0.3
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.3
 sphinxcontrib-serializinghtml==1.1.4
-urllib3==1.26.5
+urllib3==1.26.18