Consider using the stable API when building wheels for CPython

I noticed your tweet about having lots of wheels (thanks for the PyPy one BTW). Perhaps you could consider creating stable ABI wheels? I think if you add

```

[bdist_wheel]

py-limited-api = cp34

```

to the `setup.cfg` then `python setup.py bdist_wheel` will build something like `MarkupSafe-1.1.1-cp34-abi3-macosx_10_6_intel.whl` which any `cpython>-3.4` will support on macosx. This would save having to create a cpython-3.4, cpython3.5, ... wheels and would also mean you would future proof yourself for any new versions of cpython.



You might need to adjust the CI wheel build to only build the wheel once, since the various versions of CPython will build a wheel with the same name, I think `bdist_wheel` is unhappy if a wheel already exists.

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