Bump pip from 20.2.4 to 20.3 in /.github/workflows (cjolowicz#690)

Bumps [pip](https://github.com/pypa/pip) from 20.2.4 to 20.3.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/master/NEWS.rst)
- [Commits](pypa/pip@20.2.4...20.3)

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

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

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
-pip==20.2.4
+pip==20.3
 cookiecutter==1.7.2
 nox==2020.8.22
 nox-poetry==0.5.0