Adds pytest version to fix travis build errors

Pipfile

@@ -3,7 +3,7 @@
 # update these accordingly
 
 [dev-packages]
-pytest="*"
+pytest=">=3.6"
 wheel="*"
 pytest-cov="*"
 PyYAML = "*"

dev-requirements.txt

@@ -1,6 +1,6 @@
 # Note that there is also a Pipfile for this project if you are updating this 
 # file do not forget to update the Pipfile accordingly
 pyyaml
-pytest
+pytest>=3.6
 wheel
 pytest-cov