title: pytest ImportError
body: <p>My folder layout is roughly like this</p>
  
  <pre><code>mesa/
      └── mesa
          └── __init__.py
          └── some module files
      └── tests
  </code></pre>
  
  <p>When I run <code>pytest --cov=mesa tests/</code> in the topmost mesa folder, I received the following error messages (only a snippet)</p>
  
  <pre><code>==================================== ERRORS ====================================
  __________________ ERROR collecting tests/test_batchrunner.py __________________
  ImportError while importing test module '/Users/kumom/Documents/Projects/mesa/tests/test_batchrunner.py'.
  Hint: make sure your test modules/packages have valid Python names.
  Traceback:
  tests/test_batchrunner.py:8: in &lt;module&gt;
      from mesa import Agent, Model
  E   ImportError: cannot import name 'Agent'
  </code></pre>
  
  <p>Why would this happen even when <code>__init__.py</code> file exists?</p>