Get rid of __future__ imports
Review Request #11944 — Created Jan. 20, 2022 and submitted
Now that Djblets is Python 3.7+, we don't need to import stuff from
__future__in every file. This change does that, and removes some old
leftover per-file copyright headers while I'm in there.
Ran unit tests.
Revision 2 (+98 -3428)
Checks run (2 succeeded)