merge (some) Github suggested ignores

From https://github.com/github/gitignore/blob/master/Python.gitignore

Merged in "compiled" and "distribution/packaging" sections. There are
other things in the example we might want too; I'm not knowledgeable
enough to judge.

Turns out a directory pattern, e.g. `__pycache__/` need only be
specified once. Probably means some of the Leo specific excludes can be
simplified as well.