Bump pip from 20.3.3 to 21.1 (#14)

Bumps [pip](https://github.com/pypa/pip) from 20.3.3 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@20.3.3...21.1)

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

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

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

requirements.txt

@@ -34,7 +34,7 @@ mypy-extensions==0.4.3
 packaging==20.8
 PasteDeploy==2.1.1
 pathspec==0.8.1
-pip==20.3.3
+pip==21.1
 plaster==1.0
 plaster-pastedeploy==0.7
 pluggy==0.13.1