setup.py: Remove use of "imp" module

[douglas-raillard-arm]

Python 3.12 removed the "imp" module, so replace its use in devlib.

closes #644

[marcbonnici]

As a completely naïve question without looking into actual implementation details.
I was wondering if there would be any benefit in using the example replacement provided in the Python release notes [1] both in devlib and referring to this from WA?

Otherwise I've tested the PR and this looks like it does the job.

[1] https://docs.python.org/dev/whatsnew/3.12.html#imp

[douglas-raillard-arm]

Yes and no. The snippet they provide is:

import importlib.util
import importlib.machinery

def load_source(modname, filename):
    loader = importlib.machinery.SourceFileLoader(modname, filename)
    spec = importlib.util.spec_from_file_location(modname, filename, loader=loader)
    module = importlib.util.module_from_spec(spec)
    # The module is always executed and not cached in sys.modules.
    # Uncomment the following line to cache the module.
    # sys.modules[module.__name__] = module
    loader.exec_module(module)
    return module

That is closer to how an import using the import statement works, but it's not even fully accurate:

• Relative imports in the module won't work (unless you uncomment the sys.modules assignment, despite the comment only stating something about caching, it's actually "mandatory caching")
• Circular dependencies won't work (ditto)
• return module is actually wrong, it should use return sys.modules[module.__name__] as some modules manipulate sys.modules[__name__] and the import statement does honor that.

So all that to say that if you want a properly "compliant" way of doing it, it can be quite tricky, and on top of that these APIs have changed approximately every 5 minutes, meaning that it's unlikely this snippet will stay "the best way to do it" for more than a couple years (look for "deprecated" in https://docs.python.org/3/library/importlib.html).

OTH, the exec() will always stay wrong and simple, and if something breaks it will be easy to understand why for anyone without being familiar with the intricacies of Python import system (e.g. no __name__ defined when running the module, it can just be added in the globals like I did for __file__).

referring to this from WA?

If you mean the plugin import system in WA, I wouldn't do that. Plugins expect to be proper Python modules so they should be imported the proper way. For version_helper.py, we have full control on what the module does, and we can easily tailor its import to its content if needed, we don't have this luxury with 3rd party modules.