Adjust the tests for the `pathlib.types` module so that they can be run
against the `pathlib-abc` PyPI package, which is a backport of the module
for older Python versions.
Specifically, we add a `.support.is_pypi` switch that is false in the
stdlib and true in the pathlib-abc package. This controls which package
we import, and whether or not we run tests against `PurePath` and `Path`.
For compatibility with older Python versions, we stop using
`zipfile.ZipFile.mkdir()` and `zipfile.ZipInfo._for_archive()`.
Test Posix-flavoured `pathlib.types._JoinablePath` in a dedicated test
module. These tests cover `LexicalPosixPath`, `PurePosixPath` and
`PosixPath`, where `LexicalPosixPath` is a simple implementation of
`_JoinablePath` for use in tests.
Barney Gale