Bump pip from 9.0.1 to 21.1

Bumps [pip](https://github.com/pypa/pip) from 9.0.1 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@9.0.1...21.1)

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

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

requirements_dev.txt

@@ -1,4 +1,4 @@
-pip==9.0.1
+pip==21.1
 bumpversion==0.5.3
 wheel==0.30.0
 watchdog==0.8.3