We don't support Python 2 anymore so we don't need this
compatibility library.
six.reraise usages are left as is until it'll be moved to some
base lib like oslo.utils to not re-implenent this method in
Horizon.
This patch also removes Python2-specific base test case methods
assertItemsEqual and assertNotRegexpMatches in flavor of new
Python 3 analogues.
Change-Id: I0b567382edf4d68674a7b8d0b02333fb57293958
e976461d85