app: avoid sys.version_info named tuple

87f921c26c9bad7fc5176939a9438125e43b31b0 broken git-cola on Snow
Leopard.  Fix it by using the more portable
sysconfig.get_python_version() instead of relying on sys.version_info
being a named-tuple.

Reported-by: Uri Okrent <uokrent@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>