CI failures related to installing homebrew python

[mroch]

I've started getting build failures from both Travis and Circle that I think are related to #26287.

==> Pouring [email protected]_3.sierra.bottle.1.tar.gz
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/2to3-2
Target /usr/local/bin/2to3-2
is a symlink belonging to python. You can unlink it:
  brew unlink python
To force the link and overwrite all conflicting files:
  brew link --overwrite python@2
To list all files that would be deleted:
  brew link --overwrite --dry-run python@2
Possible conflicting files are:
/usr/local/bin/2to3-2 -> /usr/local/Cellar/python/2.7.14/bin/2to3-2
/usr/local/bin/2to3-2.7 -> /usr/local/Cellar/python/2.7.14/bin/2to3-2.7
/usr/local/bin/idle2 -> /usr/local/Cellar/python/2.7.14/bin/idle2
...

https://travis-ci.org/facebook/flow/jobs/364227445
https://circleci.com/gh/facebook/flow/1954

seems like it's already using a homebrew python and failing while trying to overwrite with a different version.

[ilovezfs]

Run brew upgrade python as a first step and the problem should go away.

[ilovezfs]

brew uninstall python may also work.

[mroch]

@ilovezfs this isn't a bug? seems like it's trying to upgrade from python 2.7.14 to 2.7.14_3, and it's doing it automatically as a dependency of installing something else, and the instructions don't mention either of your suggestions, so it seems like there's room for improvement here

[ilovezfs]

@mroch nope not a bug. The python formula was upgraded to be Python 3 and a newly added formula, python@2, is now Python 2. So if you have something that depends on the new python@2, but you haven't upgraded your python formula yet, then there will be a conflict between that outdated python installation and the new python@2 formula.