Fix: import fails due to pytest ModuleNotFoundError

`import pecos` currently fails due to `pytest` being called on this line: https://github.com/sandialabs/pecos/blob/e0aea99535f92f6f63f38387a79898ccb6966f37/pecos/graphics.py#L423

This commit moves pytest up to the required section which allows pytest to be installed during the `pip` installation step.

requirements.txt

@@ -3,6 +3,7 @@ pandas
 numpy
 jinja2
 matplotlib
+pytest
 
 # Optional
 pvlib
@@ -15,5 +16,4 @@ sphinx
 sphinx_rtd_theme
 
 # Testing
-pytest
 coverage