wheel now uses flit_core as its build backend. We can't use %py3_build
or %py3_install anymore, as there is no setup.py. This commit reworks
the bootstrap logic accordingly. The build process requires building a
wheel anyways, so python3-wheel-wheel is always built.
When bootstrapping, neither pip and thus pyproject-rpm-macros is not
available so the following workarounds are employed:
- Build wheel using `%python3 -m flit_core.wheel` instead of
%pyproject_wheel which requires pip
- Manually unpack wheel and create entrypoints instead of
%pyproject_install which requires pip.
