In the abstract interface of `JoinablePath`, replace `__str__()` with
`__vfspath__()`. This frees user implementations of `JoinablePath` to
implement `__str__()` however they like (or not at all.)
Also add `pathlib._os.vfspath()`, which calls `__fspath__()` or
`__vfspath__()`.
Follow-up to fbffd70. Set `encoding='utf-8'` when reading and writing text
in the tests for the private pathlib ABCs, which allows the tests to run
with `-W error -X warn_default_encoding`
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 `pathlib.types._WritablePath` in a dedicated test module. These tests
cover `WritableZipPath`, `WritableLocalPath` and `Path`, where the former
two classes are implementations of `_WritablePath` for use in tests.
Test `pathlib.types._ReadablePath` in a dedicated test module. These tests
cover `ReadableZipPath`, `ReadableLocalPath` and `Path`, where the former
two classes are implementations of `_ReadablePath` for use in tests.
