Bump pip from 19.2.1 to 21.1 (#52)

Bumps [pip](https://github.com/pypa/pip) from 19.2.1 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@19.2.1...21.1)

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

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

Author: dependabot[bot]

requirements_dev.txt

@@ -1,4 +1,4 @@
-pip==19.2.1
+pip==21.1
 wheel==0.33.4
 six==1.12.0
 Sphinx==1.8.5