Fix tempfile reset code & test on newer Python

Python 2.7.3 (rc status) and 3.2.3/3.3 (rc, respectively alpha status)
have fixed http://bugs.python.org/issue12856 which we worked around
ourselves.

This means two things:

- we don't need to manually reset the module
- we can't test for the no-reset case

Unfortunately current Debian Sid has the 2.7.3 RC but still labeled as
2.7.2+ ☹️

, so we have to manually do an extra check (in the unit test
only); I expect Debian will update to official 2.7.3 as soon as it's
released, and then we can remove this override.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>