setup: drop support for Python 3.5
We talked about this on the mailing list [1] and there seemed to be
agreement that Python 3.5 is effectively dead and no longer worth
supporting.
So this commit changes our minimum version requirement to 3.6.2.
[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2022-February/147885.html
Differential Revision: https://phab.mercurial-scm.org/D12315
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed