Adds pytest version to fix travis build errors
MODIFY
Pipfile
Pipfile
# update these accordingly
[dev-packages]
-pytest="*"
+pytest=">=3.6"
wheel="*"
pytest-cov="*"
PyYAML = "*"

MODIFY
dev-requirements.txt
dev-requirements.txt
# 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

Adds pytest version to fix travis build errors