title: brew install doesn't link python3
body: <p>I'm having trouble linking python3 and making python3 the default python.</p>
  
  <p><code>MacOS 10.13.6</code></p>
  
  <p>Here's what I did:</p>
  
  <pre><code>$ python --version
  Python 2.7.15
  $ python3 --version
  Python 3.7.0
  $ xcode-select --version
  xcode-select version 2349.
  $ brew install python
  ...
  
  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
  ...
  </code></pre>
  
  <p>It doesn't work. <code>python</code> is still 2.*</p>
  
  <pre><code>$ python --version
  Python 2.7.15
  $ python3 --version
  Python 3.7.0
  $ pip --version
  pip 10.0.1 from /usr/local/lib/python2.7/site-packages/pip (python 2.7)
  $ pip3 --version
  pip 18.0 from /usr/local/lib/python3.7/site-packages/pip (python 3.7)
  </code></pre>
  
  <p>So I did:</p>
  
  <pre><code>$ brew link python3
  Warning: Already linked: /usr/local/Cellar/python/3.7.0
  To relink: brew unlink python &amp;&amp; brew link python
  $ python --version
  Python 2.7.15
  $ brew install python3
  ...
  
  Warning: python 3.7.0 is already installed and up-to-date
  To reinstall 3.7.0, run `brew reinstall python`
  $ brew reinstall python
  </code></pre>
  
  <p>Also doesn't work</p>
  
  <pre><code>$ brew link python3
  Warning: Already linked: /usr/local/Cellar/python/3.7.0
  To relink: brew unlink python &amp;&amp; brew link python
  $ python --version
  Python 2.7.15
  $ python3 --version
  Python 3.7.0
  </code></pre>