bpo-33178: Add BigEndianUnion, LittleEndianUnion classes to ctypes (GH-25480)

* bpo-33178: Add BigEndianUnion, LittleEndianUnion classes to ctypes
* GH-25480: remove trailing whitespace in ctypes doc
* GH-25480: add news entry blurb
* GH-25480: corrected formatting error in news blurb
* GH-25480: simplified, corrected formatting in news blurb
* GH-25480: remove trailing whitespace in news blurb
* GH-25480: fixed class markup in news blurb
* GH-25480: fixed unsupported type tests and naming per review comments
* GH-25480: fixed whitepace errors
* condensed base class selection for unsupported byte order tests
* added versionadded tags for new EndianUnion classes