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