setup: allow py3 install without env vars
5.2 is the first release of Mercurial where py3 support is expected to be
widely used, therefore we should allow installing it without hoop-jumping.
Differential Revision: https://phab.mercurial-scm.org/D7151
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.