Use new minimum version of pytest

pytest-cov requires pytest>=4.6 so we explicitly list that as a dev
dependency.

Pipfile

@@ -1,9 +1,9 @@
 # Note that there are a dev-requirements.txt and a docs/doc-requirements.txt
-# files in this project, if you are updating this Pipfile make sure to 
+# files in this project, if you are updating this Pipfile make sure to
 # update these accordingly
 
 [dev-packages]
-pytest=">=3.6"
+pytest=">=4.6"
 wheel="*"
 pytest-cov="*"
 PyYAML = "*"

dev-requirements.txt

@@ -1,7 +1,7 @@
 # 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>=3.6
+pytest>=4.6
 wheel
 pytest-cov
 pre-commit