python3.8: bulk removal of now obsolete "_python38" packages. (#9328)

* 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.

pyproject_hooks: new recipe. (#8281)

This will be needed when updating the `build` package to version
0.10.0, as it changes dependencies from `pep517` to this one.