Revert "build: Make Python a required dependency."

This reverts commit a2e8238b.

Reason for revert: breaks build on systems that symlink
/usr/bin/python to Python 2 rather than Python 3 -- for example,
Ubuntu 18.04.

Change-Id: Ia58fb4d9b337e65d0a7118ecbaa7e22e4b4619c2