closes bpo-35184: Fix XML_POOR_ENTROPY option that breaks makesetup parsing of pyexpat line in Setup. (GH-13064)

When the line is uncommented, the equals character causes it to be incorrectly interpreted
as a macro definition by makesetup.  This results in invalid Makefile output.

The expat code only requires XML_POOR_ENTROPY to be defined; the value is unnecessary.