fix: use 'six.wraps' vs. 'functools.wraps' (#37)
fix: use 'six.wraps' vs. 'functools.wraps' (#37)

'six.wraps' preserves metdata better under Python 2.7, which makes
stuff like pytest fixture injection work with plain functions.

Closes #36.
googleapis/python-test-utils,googleapis/python-test-utils