Initial support for IPv6 (#8225)

- Merge rabit socket into XGBoost.
- Dask interface support.
- Add test to the socket.

doc/conf.py

@@ -204,7 +204,7 @@ latex_documents = [
 ]
 
 intersphinx_mapping = {
-    "python": ("https://docs.python.org/3.6", None),
+    "python": ("https://docs.python.org/3.8", None),
     "numpy": ("https://docs.scipy.org/doc/numpy/", None),
     "scipy": ("https://docs.scipy.org/doc/scipy/reference/", None),
     "pandas": ("https://pandas.pydata.org/pandas-docs/stable/", None),