Bump pip from 20.1 to 21.1 (#1642)

Bumps [pip](https://github.com/pypa/pip) from 20.1 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@20.1...21.1)

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

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

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
-pip==20.1
+pip==21.1
 Sphinx==3.2.1
 blurb
 polib