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/gurobipy/linux.nix

@@ -5,7 +5,8 @@ let utf =
   else if python.ucsEncoding == 4 then "32"
   else throw "Unsupported python UCS encoding UCS${toString python.ucsEncoding}";
 in python.pkgs.buildPythonPackage
-  { name = "gurobipy-7.5.2";
+  { pname = "gurobipy";
+    version = "7.5.2";
     src = fetchurl
       { url = "http://packages.gurobi.com/7.5/gurobi7.5.2_linux64.tar.gz";
         sha256 = "13i1dl22lnmg7z9mb48zl3hy1qnpwdpr0zl2aizda0qnb7my5rnj";