Bump pip from 19.2.3 to 21.1

Bumps [pip](https://github.com/pypa/pip) from 19.2.3 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](https://github.com/pypa/pip/compare/19.2.3...21.1)

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

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

requirements.txt

@@ -1,4 +1,4 @@
-pip==19.2.3
+pip==21.1
 bump2version==0.5.11
 wheel==0.33.6
 watchdog==0.9.0

requirements_dev.txt

@@ -1,4 +1,4 @@
-pip==19.2.3
+pip==21.1
 bump2version==0.5.11
 wheel==0.33.6
 watchdog==0.9.0