Remove PYO3_NO_PYTHON hack for pyo3 >= 0.16.4

[messense]

See #830 (comment)

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[messense]

@ravenexp Only the Docker tests failed now:

error: cannot set a minimum Python version 3.7 higher than the interpreter version 3.6 (the minimum Python version is implied by the abi3-py37 feature)

https://github.com/PyO3/maturin/runs/5847881580?check_suite_focus=true

[ravenexp]

I'm confused about where does Python 3.6 come from. pyo3 v0.16 only supports Python 3.7 and newer.

[messense]

The default Python is 3.6 in that Docker image, but it shouldn't matter given that it's building an abi3 wheel?

[ravenexp]

PyO3 does not support setting minimum Python version for abi3 lower than 3.7 anymore.

Edit: I think the issue here is that unless PYO3_NO_PYTHON is set, the build script still tries to detect the host Python version, and refuses to use it when it's older than the minimum supported version.

[messense]

I think the issue here is that unless PYO3_NO_PYTHON is set, the build script still tries to detect the host Python version, and refuses to use it when it's older than the minimum supported version.

Yeah, I don't think it makes sense for abi3 wheel on Unix now and on Windows once we integrate python3-dll-a.

[ravenexp]

Yeah, I don't think it makes sense for abi3 wheel on Unix now and on Windows once we integrate python3-dll-a.

This doesn't make much sense to me, but it's the long existing behavior someone may depend on.
It can be easily changed after PyO3/pyo3#2276 got merged, but this probably needs more discussion as a separate PyO3 issue.

[ravenexp]

@ravenexp Only the Docker tests failed now:

error: cannot set a minimum Python version 3.7 higher than the interpreter version 3.6 (the minimum Python version is implied by the abi3-py37 feature)

https://github.com/PyO3/maturin/runs/5847881580?check_suite_focus=true

This should also work after upgrading pyo3 to 0.16.4