Multi-phase module initialisation (PEP 489)

[scoder]

CPython 3.5 introduced a new module initialisation scheme in multiple phases:
https://www.python.org/dev/peps/pep-0489/

This improves the support for extension module reloading and could also allow running extension modules as main module (python -m) in the future (probably Py3.7).

Worth implementing. It requires splitting up the generate_module_init_func() method in https://github.com/cython/cython/blob/master/Cython/Compiler/ModuleNode.py to generate separate functions for the separate phases, and then also to find a compatible fallback way to execute them in older Python versions.

The proposed CPython support for runnable extension modules is tracked here: https://bugs.python.org/issue30403

[scoder]

Implemented in #1794

[scoder]

Reopening since the implementation in 0.27.0 is not complete and was disabled in 0.27.1. It requires proper reloading support, which is not trivial.

[scoder]

The implementation in the latest master branch should be safer now. It does not allow module reloading nor does it support subinterpreters. In fact, it detects subinterpreters and prevents a reload in a different one than the first import. A reload in the same interpreter simply returns the existing module.