wrapper: replace usage of deprecated imp module for Python 3

A warning is emitted

  DeprecationWarning: the imp module is deprecated in favour of
  importlib; see the module's documentation for alternative uses

Change-Id: I6c5a9e024a9a904e02a24331f615548be3fe5f8e
Signed-off-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/230984
Reviewed-by: Jonathan Nieder <jrn@google.com>