fix(sitemap): guard the unavailable ExpatParser.flush on older CPython patch releases

[Mantisus]

Description

• _XmlSitemapParser.flush called ExpatParser.flush() unguarded. The method was added in CPython 3.10.14, 3.11.9, and 3.12.3, so on earlier patch releases every sitemap parse logged a spurious WARNING. It's now called only when it exists.
• Moved the yield loop into a finally, so a raising flush() no longer discards the items the handler has already collected.

Issues

• Closes: Unguarded ExpatParser.flush() logs a spurious warning on Python < 3.10.14 #2119

Testing

• Added tests for a parser without flush and for a failing flush().