Update to Sphinx 4

- allow local development using editable = true in Pipfile and by adding
  a setup.py
- bump dependencies for Sphinx 4.x

Pipfile

@@ -4,12 +4,14 @@ url = "https://pypi.org/simple"
 verify_ssl = true
 
 [dev-packages]
-sphinx = ">=3"
-ipcalc = ">=1.99"
 coverage = "*"
 twine = "*"
 path = "*"
 pytest = "*"
 
 [packages]
-jandd-sphinxext-ip = {path = "."}
+jandd-sphinxext-ip = { path = ".", editable = true }
+Sphinx = ">=4"
+docutils = "*"
+six = "*"
+ipcalc = ">=1.99"