contrib: update to latest Python 2.7, 3.7, and 3.8
We would ideally update the 3.5 and 3.6 versions as well. But Python
didn't generate exe installers for newer versions for some reason.
Differential Revision: https://phab.mercurial-scm.org/D8466
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.