mac: Need to handle Homebrew's removal of Python 2 support

[EricCousineau-TRI]

Slack conversation: https://drakedevelopers.slack.com/archives/C270MN28G/p1567516112017400

jamiesnape Today at 9:08 AM
Actually, Homebrew are removing Python 2 support from almost everything.

jwnimmer-tri 3 hours ago
Though probably obvious, I would support dropping Drake Python 2 support in macOS two months early, if the alternative is any substantial amount of work.

jwnimmer-tri 3 hours ago
Possibly with a quick user survey first, and/or work-around instructions on how users could pin to the last-good versions of Drake and homebrew.

jamiesnape 2 hours ago
Pinning Homebrew without breaking something is basically impossible. We may be able to work around with pip in the interim, though we would need to be pretty careful.

eric.cousineau 1 hour ago
My vote would also be to drop Python 2 on Mac. If Homebrew isn't supporting it, we shouldn't bend over backwards either?
That being said, would we need to accelerate releasing our flavor of Director with Python 3?

jamiesnape 1 hour ago
For director to actually be useful, yes. Dropping Python 2 support drops the various lcm types.

eric.cousineau 18 minutes ago
Do you have a rough estimate of what it'd take to try and use pip for Python 2 (keep old cruft alive) vs. just move to Python 3 wholesale on Mac (new hotness)?

jamiesnape 10 minutes ago
Not really. If the pip packages just work, that fix is trivial; if not it depends why, and honestly I can’t predict any failure without trying. Switching to Python 3 only is non-trivial, but it is work that we would have to do anyway.

eric.cousineau 9 minutes ago
Gotcha. How would you feel about timeboxing the pip fallback to perhaps an hour, and if it doesn't go smoothly, we bite the bullet and upgrade?

jamiesnape 7 minutes ago
Sounds good.

[jamiesnape]

Watching Homebrew/homebrew-core#42494.

EDIT(eric): Will need mitigation once that watched PR gets merged.

[EricCousineau-TRI]

Given timing of #10606 to drop Python 2 Mac support, this may become not-a-problem.

[EricCousineau-TRI]

Just going to close this for now, in lieu of tracking #12046.