pythonPackages.av: disable for python2

Uses shlex commands which aren't present in python2
```
      from shlex import quote
  ImportError: cannot import name quote
```

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

@@ -1,6 +1,7 @@
 { lib
 , buildPythonPackage
 , fetchPypi
+, isPy27
 , numpy
 , ffmpeg_4
 , pkgconfig
@@ -9,6 +10,7 @@
 buildPythonPackage rec {
   pname = "av";
   version = "8.0.2";
+  disabled = isPy27; # setup.py no longer compatible
 
   src = fetchPypi {
     inherit pname version;