spec: Use Python 3 for building when possible
spec: Use Python 3 for building when possible

While RHEL / CentOS are still using Python 2 for the time being,
Fedora has already switched to Python 3 as the default Python
interpreter a while ago, so on that OS it doesn't make sense to
drag in Python 2 anymore; the same applies to future RHEL versions.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

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