Use implemenation-specific I/O open / constructors in tests

[cmaloney]

Bug report

Bug description:

Most I/O tests are setup for running on multiple implementations. As tests have been added for bug fixes some now use open() or the class constructor directly instead of the self. version which does implementation delegation. This means that some features are not tested when the I/O module implementation is changed (ex. _io, _pyio).

I'm planning to use this as a meta issue as I find individual cases and update the test, fixing up implementations as needed to get tests passing.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-133982: Update test_bufio to use self.open #133983