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`.
This is a squash commit of the joint work from:
* Jan Tojnar (@jtojnar)
* Linus Heckemann (@lheckemann)
* Ryan Mulligan (@ryantm)
* romildo (@romildo)
* Tom Hunger (@teh)
