Version 5.x of traitlets requires Python 3.7, so it doesn't build on Python
3.6. This commit downgrades traitlets to 4.3.3 on Python versions older than
3.7 so that it can still be used.
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`.
