Switch to install from .whl package.
IMO, we should do the same whenever possible for Python packages
that do not require compilation, or otherwise special needs.
It gives us some advantages:
- faster builds.
- simpler recipes.
- can get/compare SHA_256 right from PyPI download page.
This one is also valid for tarballs's case, if we use
pypi.io / pythonhosted.org URIs of course.
- can help avoid complex build dependencies (like `poetry`).
The last one a major one, as it can help reduce maintenance costs.
* python3.8: bulk removal of now obsolete "_python38" packages.
Done via scripting. No functional change intended or expected.
Some manual tweaks will follow.
* python3.8: further manual tweaks after bulk-3.8-removal.
Unlike the previous commit with automated changes,
I've tested builds for *these* recipes in beta4 64 bits.
