setup: install python3 over python

When installing python in Ubuntu 20.04, apt will install
python-is-python2. This is incompatible with the repo utility, which
requires at least python3.6. Subsequent uses of repo will fail with
an obscure "Syntax Error" error message.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Dhruv Tripathi <dhruv.tripathi@arm.com>

Change-Id: Id7cf2c09629a73afa0e038970916253154eedc6c