This isn't install by default on Mac, which causes a test failure. The logic
for avoiding the command is a little goofy, but nested #if isn't supported,
and it still seems worth running the hg command to see if anything explodes.
With this, the py3 tests run (almost) cleanly on 10.14.6:
- Ran 835 tests, 58 skipped, 1 failed.
Alas, the mac-packaging test is skipped because it's slow. The failure here is
in test-releasenotes-merging.t, complaining about not being able to import the
fuzzywuzzy module. I have it installed on py3 (thus the test isn't skipped),
but not on py2. So there must be some unintended cross pollination here when
running hg commands.