bpo-31801: Enum: add _ignore_ as class option (#5237)

* bpo-31801:  Enum:  add _ignore_ as class option

_ignore_ is a list, or white-space seperated str, of names that will not
be candidates for members; these names, and _ignore_ itself, are removed
from the final class.

* bpo-31801:  Enum:  add documentation for _ignore_

* bpo-31801: Enum: remove trailing whitespace

* bpo-31801: Enum: fix bulleted list format

* bpo-31801: add version added for _ignore_