Fix deprecations and modernize testing

- use pytest and tox for testing
- fix deprecation warnings from Sphinx and Docutils

Pipfile

@@ -4,14 +4,14 @@ url = "https://pypi.org/simple"
 verify_ssl = true
 
 [dev-packages]
-sphinx = ">=2"
-path-py = ">=8.2.1"
+sphinx = ">=3"
 ipcalc = ">=1.99"
 coverage = "*"
 twine = "*"
+tox = "*"
+pytest = "*"
+path = "*"
+tox-pipenv = "*"
 
 [packages]
 jandd-sphinxext-ip = {editable = true,path = "."}
-
-[requires]
-python_version = "3.7"