Bump pip from 20.2.2 to 21.1 in /docs

Bumps [pip](https://github.com/pypa/pip) from 20.2.2 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](https://github.com/pypa/pip/compare/20.2.2...21.1)

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

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

docs/requirements_docs.txt

@@ -1,4 +1,4 @@
-pip==20.2.2
+pip==21.1
 bump2version==1.0.0
 wheel==0.35.1
 argh