Package rpm-build-python3: Information

- define %__python3 as full path /usr/bin/python3

- check-python3-provs-importable.sh included
  (to manually check Python3 packages, say, in hasher;
  https://lists.altlinux.org/pipermail/devel/2017-February/202384.html )

- Assume that setuptools are required if the traditional Python3 build/install
  macros are used (overridable through %python3_setup_buildrequires).

  The idea is that the Build Root Policy scripts for Python3 are executed
  unconditionally whenever python3 is present in the system, but the
  setuptools requirement is optional. (As a consequence, there may be a
  working system with Python3 without setuptools, because setuptools
  are not used at runtime.)