README

setup: fix mistake that prevented Python 3 from being excluded

My intent was to only allow Python 3 if the right environment variable
is set (for when people want to use `pip install .` on hg
locally). This fixes the bug in my previous change. I verified that
`python3.6 run-tests.py` still passes the tests that passed before,
and that all tests pass on 2.7 (including our virtualenv-using
installation test).

Differential Revision: https://phab.mercurial-scm.org/D185

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.