Bump pip from 20.2.4 to 20.3.1 in /.github/workflows (#87)

Bumps [pip](https://github.com/pypa/pip) from 20.2.4 to 20.3.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/master/NEWS.rst)
- [Commits](https://github.com/pypa/pip/commits)

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

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

{{cookiecutter.project_name}}/.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
-pip==20.2.4
+pip==20.3.1
 pre-commit==2.9.0
 black==20.8b1
 flake8==3.8.4