importlib.machinery.ModuleSpec and find_spec() were introduced in Python 3.4
under PEP 451. They replace the find_module() API of PEP 302, which was
deprecated from Python 3.4. They were removed in Python 3.12 along with the
imp module.
This change adds support for the PEP 451 APIs. Mitogen should no longer import
imp on Python versions that support ModuleSpec. Tests have been added to cover
the new APIs.
CI jobs have been added to cover Python 3.x on macOS.
Refs #1033
Co-authored-by: Witold Baryluk <witold.baryluk@gmail.com>
Alex Willmer