docs: remove six from the intersphinx mappings (#17165)

002f77c removed the last reference to
six, so there is no need to pull down the six docs inventory

Author: mr-c

docs/source/conf.py

@@ -266,7 +266,6 @@
 
 intersphinx_mapping = {
     "python": ("https://docs.python.org/3", None),
-    "six": ("https://six.readthedocs.io", None),
     "attrs": ("https://www.attrs.org/en/stable/", None),
     "cython": ("https://docs.cython.org/en/latest", None),
     "monkeytype": ("https://monkeytype.readthedocs.io/en/latest", None),