bpo-17239: Disable external entities in SAX parser (GH-9217)

bpo-17239: Disable external entities in SAX parser (GH-9217)

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