Fix infinite recursion for aliases in pkgs/top-level/python-aliases.nix

This fixes #750.

lib.nix

@@ -17,7 +17,16 @@ let
     lib.strings.toLower (lib.strings.concatStringsSep "-" partsWithoutSeparator);
 
   # Normalize an entire attrset of packages
-  normalizePackageSet = lib.attrsets.mapAttrs' (name: value: lib.attrsets.nameValuePair (normalizePackageName name) value);
+  normalizePackageSet = packageSet: lib.attrsets.mapAttrs'
+    (name: value:
+      let normalizedName = normalizePackageName name; in
+      if (builtins.hasAttr normalizedName packageSet) then
+      # Prevent infinite recursion for aliases in nixpkgs/pkgs/top-level/python-aliases.nix
+        lib.attrsets.nameValuePair name value
+      else
+        lib.attrsets.nameValuePair normalizedName value
+    )
+    packageSet;
 
   # Get a full semver pythonVersion from a python derivation
   getPythonVersion = python: