Bump pip from 18.1 to 19.2 in /mozdef_util (#1718)

Bumps [pip](https://github.com/pypa/pip) from 18.1 to 19.2.
- [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/18.1...19.2)

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

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

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

mozdef_util/requirements_dev.txt

@@ -1,4 +1,4 @@
-pip==18.1
+pip==19.2
 bumpversion==0.5.3
 wheel==0.34.2
 watchdog==0.9.0