Bump pip from 19.1.1 to 21.1 (#8)

Bumps [pip](https://github.com/pypa/pip) from 19.1.1 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@19.1.1...21.1)

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

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

requirements_dev.txt

@@ -2,7 +2,7 @@ bump2version==0.5.10
 coverage==4.5.3
 flake8==3.7.7
 mock==3.0.5
-pip==19.1.1
+pip==21.1
 pytest==4.5.0
 Sphinx==1.8.5
 tox==3.13.1