setup.py: default universal wheel

By default, flag that a universal wheel will be built when a maintainer
generated a release package via `bdist_wheel`. This avoids the need to
specify the `--universal` option in the command line.

Signed-off-by: James Knight <james.d.knight@live.com>

setup.cfg

@@ -0,0 +1,3 @@
+[bdist_wheel]
+# build a universal wheel (Python 2 and 3 supported)
+universal=1