v0.18.3
Backport fix for bpo-38804
Fix bug in super() with metaclasses
Add tkFileDialog to future.movers.tkinter
Merge in pending v0.16.x changes
add back in py2.6
Merge pull request #582 from r3m0t/patch-6
Add PY37_PLUS, PY38_PLUS, and PY39_PLUS
Prepare for 0.18.3 release