pyenv is your friend. It manages the installation of multiple versions, and you can choose on a per-pipenv basis which one you want to use. A certain codebase requires 2.7.10, not 2.7.11? Pin it to 2.7.10 and it'll stay there forever, even if you upgrade the system or homebrew python.
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