PyO3: adjust mingw support to use the limited ABI again

[lazka]

Just a tracking issue.

We initially added upstream support in PyO3/pyo3#1423 but since then we gained support for the limited ABI in #148

We should try to adjust the upstream code to use the limited ABI again, so there is less difference compared to official CPython, maybe the PR can be completely reverted even.

[lazka]

Turns out this is already fixed upstream (maybe by accident?)

The abi3 feature in pyo3 is now opt-in, and mingw isn't treated separately anymore.

In maturin one can select abi3 support and I can confirm that the resulting Python extension uses the right DLL.