Bring the package into line with snapm and modern Python packaging conventions.

Confirmed with mhroncok that this is safe for a RHEL minor update:

bmr
So ... one question when you have a second I asked this in Slack #python, and got one answer (it's fine don't worry about it), but I wanted to check with you - can we safely switch from %py3_build/%py3_install to %pyproject_* in a RHEL minor update? It seemed risky to me, and I wasn't to reassured by the comments in
Fedora Python
 
mhroncok
it should be fine