pythonPackages: remove name attribute`

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`.

pkgs/development/python-modules/twisted/default.nix

@@ -1,6 +1,6 @@
 { stdenv
 , buildPythonPackage
-, fetchurl
+, fetchPypi
 , python
 , zope_interface
 , incremental
@@ -13,11 +13,10 @@
 }:
 buildPythonPackage rec {
   pname = "Twisted";
-  name = "${pname}-${version}";
   version = "18.4.0";
 
-  src = fetchurl {
-    url = "mirror://pypi/T/Twisted/${name}.tar.bz2";
+  src = fetchPypi {
+    inherit pname version;
     sha256 = "a4cc164a781859c74de47f17f0e85f4bce8a3321a9d0892c015c8f80c4158ad9";
   };