macros/python: Remove python2 macros and add pyproject macros

[EbonJaeger]

Summary

Now that Solus no longer supports anything Python 2, we can remove the old Python 2 macros.

This also addresses an old annoyance where our Python macros try to be a do-all that support PEP517 and non-PEP517 in the same invocation. Some software doesn't play nice and ship files indicating both ways, which confuses the macro, and the wrong commands are run. Making the pyproject macros explicit eliminates having to do things like rm setup.py or rm pyproject.toml during the package build.

Another very common pattern in packages is %python3_test pytest. We can shorten that by having just a %pytest macro.

Lastly, adding %python3_sitepackages% GREATLY shortens lines in package files that currently have to specify the entire path manually.

Test Plan

Build reuse with all of the new macros, and see that the resulting .eopkg file contains the correct files in the correct paths.