Add pytest plugin for automatic .coco test compilation (#888)

[ad4mf06]

Summary

This PR implements issue #888 by adding support for automatic compilation and execution of .coco files via pytest, without requiring a prior manual compilation step.

Changes

pytest_plugin.py (new file)

Created a dedicated pytest plugin containing three core functions:

• pytest_configure — Called at the start of the pytest session. Installs _PytestCoconutImporter at the front of sys.meta_path to intercept imports of .coco modules.
• pytest_collect_file — Called for each discovered file. If the file matches test_*.coco, it compiles it to a .py file alongside the source (not in a cache directory), then instructs pytest to collect that .py file. This resolves the import file mismatch error.
• pytest_ignore_collect — Prevents double-collection by ignoring the coconut_cache/ directory and any .py files that have a corresponding .coco source file.

api.py

Added a function to allow manual installation of the pytest11 entry point, enabling the plugin to be registered directly without extra configuration.

setup.py / pyproject.toml

Registered the plugin via pytest11 so it is automatically installed and activated when the package is installed.

_PytestCoconutImporter — 2 overridden methods

Inherits from CoconutImporter with two overrides:

• compile() — Replaces sys.stdin during compilation to avoid OSError caused by pytest capturing stdin.
• find_spec() — Returns a spec pointing to the compiled .py file in place, preventing the global importer from redirecting imports elsewhere.