maint: Tell git that *.py files should use python diff hunks

Git's default hunk pattern recognizer favors the C language, but it
also includes several built-in diff styles that give saner results in
other languages.  In particular, telling git to treat all .py files as
python changes the beginning of diff hunks as follows:

|  --- a/python/qemu/machine.py
|  +++ b/python/qemu/machine.py
| -@@ -337,12 +337,12 @@ class QEMUMachine:
| +@@ -337,12 +337,12 @@ def _post_shutdown(self) -> None:
|               self._qmp.close()

which makes it much easier to tell what function a patch is touching,
rather than a non-descript listing of what class contains the changes.

Sadly, our python files that don't use .py suffix (such as numerous
iotests) do not benefit from this glob.

Reported-by: John Snow <jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210215222524.1820223-1-eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>