python-discogs-client: use requests2

This fixes an issue in beets which would depend on two versions of
requests if this used requests and not requests2.

pkgs/top-level/python-packages.nix

···
       md5 = "2cc57e1d134aa93404e779b9311676fa";
     };
 
-    propagatedBuildInputs = with self; [ oauth2 requests ];
+    propagatedBuildInputs = with self; [ oauth2 requests2 ];
 
     meta = {
       description = "Official Python API client for Discogs";