Use multi-phase initialisation (PEP 489)

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Hello,

Please consider adopting multi-phase init for your Python extension modules. In short, multi-phase makes extension modules behave more like pure-Python modules and means that the import system can determine what a capabilities a module has before executing it.

Multi-phase initialisation was introduced in [PEP 489](https://peps.python.org/pep-0489/) for Python 3.5. It replaces the previous 'single-phase' mechanism by splitting the module creation process into creation and execution phases. Quoting from [the documentation](https://docs.python.org/3/c-api/module.html#multi-phase-initialization):

> Extension modules created this way behave more like Python modules: the initialization is split between the creation phase, when the module object is created, and the execution phase, when it is populated. The distinction is similar to the `__new__()` and `__init__()` methods of classes.

It is the preferred mechanism for extension module initialisation, and there is a proposal to [deprecate](https://discuss.python.org/t/89262/45) single phase init.

Thanks,
Adam

Start by reading PEP 489 and the Python extension-module initialization documentation, then locate the extension module initialization entry points in the repository. Done means the extension modules use creation and execution phases instead of single-phase initialization, with the existing module behavior preserved and relevant tests passing.

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