[3.6] bpo-17239: Disable external entities in SAX parser (GH-9217) (GH-9512)

[3.6] bpo-17239: Disable external entities in SAX parser (GH-9217) (GH-9512)

The SAX parser no longer processes general external entities by default
to increase security. Before, the parser created network connections
to fetch remote files or loaded local files from the file system for DTD
and entities.

Signed-off-by: Christian Heimes <christian@python.org>
https://bugs.python.org/issue17239.
(cherry picked from commit 17b1d5d4e36aa57a9b25a0e694affbd1ee637e45)

Co-authored-by: Christian Heimes <christian@python.org>
https://bugs.python.org/issue17239