Bump urllib3 from 1.26.9 to 1.26.18 in /docs

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.9 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@1.26.9...1.26.18)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -18,8 +18,6 @@ idna==3.3
     # via requests
 imagesize==1.3.0
     # via sphinx
-importlib-metadata==4.11.3
-    # via sphinx
 jinja2==3.1.1
     # via sphinx
 markupsafe==2.1.1
@@ -50,7 +48,5 @@ sphinxcontrib-qthelp==1.0.3
     # via sphinx
 sphinxcontrib-serializinghtml==1.1.5
     # via sphinx
-urllib3==1.26.9
+urllib3==1.26.18
     # via requests
-zipp==3.7.0
-    # via importlib-metadata