build(deps-dev): bump pip in /src/ceph-volume/plugin/zfs

Bumps [pip](https://github.com/pypa/pip) from 19.2 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...21.1)

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

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

src/ceph-volume/plugin/zfs/requirements_dev.txt

@@ -1,4 +1,4 @@
-pip==19.2
+pip==21.1
 wheel==0.30.0
 flake8==3.5.0
 tox==2.9.1