sphinx: Normalize the filename with NFC

The original problem with the normalization of the filename only
occurred because it was in NFC. However, when trying to fix it by
`mv`ing the file to a normalization-indifferent name, I used the NFD
normalized name from my file system. This means it only works on
normalizing file systems. The filename must be in the original encoding
and will be normalized by normalizing file systems like HFS+.

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

···
     sha256 = "129x3kljxq7sbg5cc12z3cmjsili6l3qrsxllkx97id1a85f7zmj";
     extraPostFetch = ''
       cd $out
-      mv tests/roots/test-images/testimäge.png \
+      mv tests/roots/test-images/testimäge.png \
         tests/roots/test-images/testimæge.png
       patch -p1 < ${./0001-test-images-Use-normalization-equivalent-character.patch}
     '';