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](pypa/pip@19.2.3...21.1)

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

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

Author: dependabot[bot]

requirements_dev.txt

@@ -2,7 +2,7 @@ aioresponses==0.6.1
 Click==7.0
 coverage==4.5.4
 flake8==3.7.8
-pip==19.2.3
+pip==21.1
 pytest==4.6.5
 pytest-aiohttp==0.3.0
 pytest-runner==5.1