title: VSCode pytest test discovery fails
body: <p>Pytest test discovery is failing. The UI states:</p>
  
  <p><code>Test discovery error, please check the configuration settings for the tests</code></p>
  
  <p>The output window states:</p>
  
  <pre><code>Test Discovery failed: 
  Error: Traceback (most recent call last):
    File "C:\Users\mikep\.vscode\extensions\ms-python.python-2019.4.11987\pythonFiles\testing_tools\run_adapter.py", line 16, in &lt;module&gt;
      main(tool, cmd, subargs, toolargs)
    File "C:\Users\mikep\.vscode\extensions\ms-python.python-2019.4.11987\pythonFiles\testing_tools\adapter\__main__.py", line 90, in main
      parents, result = run(toolargs, **subargs)
    File "C:\Users\mikep\.vscode\extensions\ms-python.python-2019.4.11987\pythonFiles\testing_tools\adapter\pytest.py", line 43, in discover
      raise Exception('pytest discovery failed (exit code {})'.format(ec))
  Exception: pytest discovery failed (exit code 3)
  </code></pre>
  
  <p>Here are my settings:</p>
  
  <pre><code>{
      "python.pythonPath": ".venv\\Scripts\\python.exe",
      "python.testing.pyTestArgs": [
          "tests"
      ],
      "python.testing.unittestEnabled": false,
      "python.testing.nosetestsEnabled": false,
      "python.testing.pyTestEnabled": true
  }
  </code></pre>
  
  <p>I can run pytest from the command line successfully FWIW.</p>