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`
Ensure that warnings about unspecified text encodings are emitted from
`ReadablePath.read_text()`, `WritablePath.write_text()` and `magic_open()`
with the correct stack level set.
When `pathlib._os.magic_open()` is called to open a path in binary mode,
raise `ValueError` if any of the *encoding*, *errors* or *newline*
arguments are given. This matches the `open()` built-in.
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.
Barney Gale