Bump pip from 21.1.2 to 21.3 in /.github/workflows

Bumps [pip](https://github.com/pypa/pip) from 21.1.2 to 21.3.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@21.1.2...21.3)

---
updated-dependencies:
- dependency-name: pip
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
-pip==21.1.2
+pip==21.3
 nox==2021.6.12
 nox-poetry==0.8.6
 poetry==1.1.6