Full Python 3.8 FFI support

[Alexander-N]

I think dgrunwald/rust-cpython#187 does provide a good starting point for us.

3.8 should also be added to CI. There was some issue when I tried to replace 3.8-dev with 3.8 in #648

Edit: Removed unrelated issue.

[kngwyu]

I wrote some patches on Python3.8 and observed no test failing...
But yeah, now it's time to check FFI compatiblitity and add it to CI.

[Alexander-N]

With github actions I get the same error as with travis:

AttributeError: module 'os' has no attribute 'link'

from https://github.com/PyO3/pyo3/runs/365982476

[kngwyu]

With github actions I get the same error as with travis:
AttributeError: module 'os' has no attribute 'link'

I saw the same error when I set up Python3.8 CI on Azure for a personal project, and ... I still don't know the solution! 😓

[davidhewitt]

Now that we have 3.8 on CI, what else are we wanting to add here? (Presumably all new C APIs added in Python 3.8?)

[Alexander-N]

Now that we have 3.8 on CI, what else are we wanting to add here? (Presumably all new C APIs added in Python 3.8?)

This and I would say checking that all changes in the c-api are accounted for. For example in dgrunwald/rust-cpython#187 there was a #[cfg(all(Py_3_5, not(Py_3_8)))] added on PyTypeObject_INIT.