Need to use fetchFromGitHub because examples/ is missing from PyPI tarball.
Also clean up the expression and specify appropriate checkInputs.
Fixes#42867.
The `buildPython*` function computes name from `pname` and `version`.
This change removes `name` attribute from all expressions in
`pkgs/development/python-modules`.
While at it, some other minor changes were made as well, such as
replacing `fetchurl` calls with `fetchPypi`.
