title: zsh: command not found: flake8 but flake8 is installed
body: <p>I'm trying to run the flake8 linter on my python code but I am having some issues. </p>
  
  <p>running pip install gives me this response: </p>
  
  <pre><code>~ pip install flake8                                                                                                                                                                                                                                  
  Requirement already satisfied: flake8 in ./Library/Python/2.7/lib/python/site-packages
  Requirement already satisfied: enum34; python_version &lt; "3.4" in ./Library/Python/2.7/lib/python/site-packages (from flake8)
  Requirement already satisfied: configparser; python_version &lt; "3.2" in ./Library/Python/2.7/lib/python/site-packages (from flake8)
  Requirement already satisfied: mccabe&lt;0.7.0,&gt;=0.6.0 in ./Library/Python/2.7/lib/python/site-packages (from flake8)
  Requirement already satisfied: pycodestyle&lt;2.4.0,&gt;=2.0.0 in ./Library/Python/2.7/lib/python/site-packages (from flake8)
  Requirement already satisfied: pyflakes&lt;1.6.0,&gt;=1.5.0 in ./Library/Python/2.7/lib/python/site-packages (from flake8)
  </code></pre>
  
  <p>but then when I try to run it I get this error.</p>
  
  <pre><code>~ flake8 --version                                                                                                                                                                                                                                    
  zsh: command not found: flake8
  </code></pre>
  
  <p>I am using a mac as well if that makes any difference. </p>
  
  <p>Please can anyone help.</p>