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.

Drop python 3.7 (and 2.7) for recipes under dev-python/

Recipes with only 3.7 are left alone. Help with #8025.

markdown-math: new recipe