build(deps): Bump pip from 7.0.3 to 21.1 in /zstackbuild/pypi_source

Bumps [pip](https://github.com/pypa/pip) from 7.0.3 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@7.0.3...21.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

zstackbuild/pypi_source/requirements.txt

@@ -18,7 +18,7 @@ numpy==1.9.2
 ordereddict==1.1
 paramiko==1.15.2
 pickleDB==0.3
-pip==7.0.3
+pip==21.1
 pycrypto==2.6.1
 pyparsing==1.5.7
 pyroute2==0.5.14