Dev install troubleshooting

Don't really know where to put this, but I just spent like half an hour and a lot of install/uninstall/banging my head against a wall trying to figure out why pytest and numpy were conflicting in weird ways until I remembered that in conda you have to usually explicitly install pytest. Figure there are a couple of other really common things that mostly don't go out of date that might be worth including.

Author: story645

doc/devel/development_setup.rst

@@ -26,7 +26,7 @@ and activated with one of the following::
    <file folder location>\Scripts\Activate.ps1  # Windows PowerShell
 
 Whenever you plan to work on Matplotlib, remember to activate the development
-environment in your shell.
+environment in your shell. 
 
 Retrieving the latest version of the code
 =========================================
@@ -65,7 +65,7 @@ also happen if you change branches) you will have to re-run
 Additional dependencies for testing
 ===================================
 This section lists the additional software required for
-:ref:`running the tests <testing>`.
+:ref:`running the tests <testing>`. 
 
 Required:
 
@@ -133,3 +133,11 @@ Optional, but recommended:
   built without Inkscape and optipng, but the build process will raise various
   warnings. If the build process warns that you are missing fonts, make sure
   your LaTeX distribution bundles cm-super or install it separately.
+  
+  .. _troubleshooting:
+  
+  Troubleshooting
+  ===============
+  * pytest - If the test runner is failing, verify that the pytest you are using 
+  is installed in your environment using the ``which pytest`` and ``which python`` 
+  commands. If the paths are not the same, install pytest in your environment.