Bump pip from 20.2.3 to 20.2.4

Bumps [pip](https://github.com/pypa/pip) from 20.2.3 to 20.2.4.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/master/NEWS.rst)
- [Commits](pypa/pip@20.2.3...20.2.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements_dev.txt

@@ -1,7 +1,7 @@
 bump2version==1.0.0
 coverage==5.3
 flake8==3.8.3
-pip==20.2.3
+pip==20.2.4
 pytest==6.1.0
 Sphinx==3.2.1
 tox==3.20.1