Add coverage for keyword arguments to warnings.warn_explicit

[seanabreau]

warnings.warn_explicit() accepts its required message, category, filename, and lineno parameters by position or keyword, but test_warnings.WarnTests does not explicitly cover those keyword calls.

This is used by pytest's Node.warn():

import warnings

warnings.warn_explicit(
    UserWarning("probe"), category=None, filename="probe.py", lineno=7
)

CPython already handles this correctly. A RustPython compatibility bug exposed the coverage gap, and a RustPython maintainer suggested adding CPython test coverage.

Add a test to the shared WarnTests class for positional, keyword, and mixed calls with both string messages and warning instances. This exercises the C and pure-Python implementations without changing runtime behavior.

AI assistance: Codex helped prepare the test and this report.

Linked PRs

• gh-158146: Test keyword arguments to warnings.warn_explicit #158147