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
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