This is un-broken as of 2018-03-09[1,2,3]. `brew install python` installs python3, but it only installs a link to this at `/usr/local/bin/python3`, not `/usr/local/bin/python` (as it did for a while).
In other words, `brew install python` now complies with PEP 394[4].
$ brew install python
…
==> Caveats
Python has been installed as
/usr/local/bin/python3
Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to
`python3`, `python3-config`, `pip3` etc., respectively, have been installed into
/usr/local/opt/python/libexec/bin
…
$ ls -l /usr/local/bin/python{,2,3}
lrwxr-xr-x 1 osteele staff 38 Mar 22 11:23 /usr/local/bin/python -> ../Cellar/python@2/2.7.14_3/bin/python
lrwxr-xr-x 1 osteele staff 39 Mar 22 11:23 /usr/local/bin/python2 -> ../Cellar/python@2/2.7.14_3/bin/python2
lrwxr-xr-x 1 osteele staff 34 Mar 31 12:27 /usr/local/bin/python3 -> ../Cellar/python/3.6.5/bin/python3
In other words, `brew install python` now complies with PEP 394[4].
[1] https://brew.sh/2018/04/09/homebrew-1.6.0/[2] https://github.com/Homebrew/homebrew-core/issues/24812
[3] https://discourse.brew.sh/t/python-and-pep-394/1813
[4] https://www.python.org/dev/peps/pep-0394/