pyacoustid: use requests2

This fixes an issue with beets which already needs requests2
due to the fetchart plugins' dependencies, so it would depend
on two different versions of requests if this package didn't
use requests2.

pkgs/top-level/python-packages.nix

···
       md5 = "b27c714d530300b917eb869726334226";
     };
 
-    propagatedBuildInputs = with self; [ requests audioread ];
+    propagatedBuildInputs = with self; [ requests2 audioread ];
 
     patches = [ ../development/python-modules/pyacoustid-py3.patch ];