bpo-37772: fix zipfile.Path.iterdir() outputs (GH-15170)

bpo-37772: fix zipfile.Path.iterdir() outputs (GH-15170)

* fix Path._add_implied_dirs to include all implied directories

* fix Path._add_implied_dirs to include all implied directories

* Optimize code by using sets instead of lists

* 📜🤖 Added by blurb_it.

* fix Path._add_implied_dirs to include all implied directories

* Optimize code by using sets instead of lists

* 📜🤖 Added by blurb_it.

* Add tests to zipfile.Path.iterdir() fix

* Update test for zipfile.Path.iterdir()

* remove whitespace from test file

* Rewrite NEWS blurb to describe the user-facing impact and avoid implementation details.

* remove redundant [] within set comprehension

* Update to use unique_everseen to maintain order and other suggestions in review

* remove whitespace and add back add_dirs in tests

* Add new standalone function parents using posixpath to get parents of a directory

* removing whitespace (sorry)

* Remove import pathlib from zipfile.py

* Rewrite _parents as a slice on a generator of the ancestry of a path.

* Remove check for '.' and '/', now that parents no longer returns those.

* Separate calculation of implied dirs from adding those

* Re-use _implied_dirs in tests for generating zipfile with dir entries.

* Replace three fixtures (abcde, abcdef, abde) with one representative example alpharep.

* Simplify implementation of _implied_dirs by collapsing the generation of parent directories for each name.