Only import imp on py2

imp is deprecatedon py3. It's also not used with py3, so just move it
to the py2 import block

Test: run `repo` command and verify warning is no longer present
Test: verify `repo sync` and `repo upload` function as expected
Change-Id: I9d59403d7819c4a478c9f54cbef114f8a96486a5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239713
Tested-by: Rashed Abdel-Tawab <rashedabdeltawab@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>