Use CPython stable APIs for implementing tuples.

[alex]

Refs #1125

Reduces the number of compilation failures with --features abi3 from 56 to 53.

I chose to implement this by simple replacing all functions with their stable equivilant. This potentially regresses performance somewhat. If it's preferable, I can switch which call is used based on the #[cfg] at the price of some verbosity.

[kngwyu]

How about making a single specific branch for this instead of merging series of PRs to the master?
If you are OK, I will create a separate branch (e.g., pyo3/abi3) and you send PRs to that branch.

[alex]

I personally find it easier to work in small PRs that can each be reviewed by themselves. This ensures that I'm constantly working off the latest version and didn't break anything.

The next PR is probably going to be type definition support, which will be the biggest part of this anyways :-)

[alex]

Coverage change is because as_slice() lost coverage since it's no longer used in iter(). Let me write a test for it.

[kngwyu]

The next PR is probably going to be type definition support, which will be the biggest part of this anyways :-)

That patch would require careful design decisions and it's easier to work for me if we have an upstream branch for it.

[alex]

Yes, I agree that PR will be very subtle and require careful thought! Is it ok to review/merge this tuple PR as is, and then we can do a branch for the type definition work?

[kngwyu]

Thanks, mostly LGTM.

[kngwyu]

Is it ok to review/merge this tuple PR as is, and then we can do a branch for the type definition work?

Of course. I'll create pyo3/abi3 branch and please open a PR to that branch first. Then let's work on the branch collaboratory.

[alex]

Ok, should be good now!

[kngwyu]

Now, https://github.com/PyO3/pyo3/tree/abi3 is available.