testing/pg_activity: upgrade to 3.0.1, testing/py3-pg_activity: remove in favour of pg_activity

pg_activity is a CLI tool that just happens to be written in Python, not a Python library, so it shouldn't use py3- prefix in the pkgname.

The py3-pg_activity aport has been added before the py_activity aport, but it's broken - missing dependencies.

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@fcolista, since your py3-pg_activity aport has been there first, do you want to maintain pg_activity?

Depends on !39953 (merged)