* 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.
Also: switch SOURCE_URI to github. The tarball there is 7.28 MB,
versus the previous 86.25 MB (due to the inclusion of mupdf.tar.gz,
which we don't need here, and weights 84.36 MB).
