[Fire162]

Provide a top-level pytest_xprocess module compatibility shim that re-exports public items from xprocess.pytest_xprocess and xprocess. This ensures downstream packages importing directly from pytest_xprocess continue to resolve without ModuleNotFoundError.

• fixes Packaging: distribution exposes xprocess but not top-level pytest_xprocess, causing downstream import errors #163

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGELOG.rst, summarizing the change and linking to the issue.
• Run pre-commit hooks and fix any issues.
• Run pytest and make sure no tests failed.