MAINT: Define Py_SETREF for pre-3.5.2 python and use in code

[eric-wieser]

No description provided.

[mattip]

We should make the backport explicit. Since Py_SETREF was added in 3.5.2 [0], this should have a #if PY_VERSION_HEX < 0x03050200 ... #else

[0] see CPython changeset 57a01d3a0ee and git tag --contains 57a01d3a0ee

[eric-wieser]

Note that to use the upstream macro, we need tp have not defined Py_limited_API or whatever it's called. I don't know if we do that or not, and didn't want to address that in this PR

[mattip]

You can check #ifdef Py_SETREF, with a comment about 3.52 and Py_LIMITED_API

[eric-wieser]

Went with your original suggestion after all - it turns out we don't use Py_LIMITED_API - which makes sense, as it didn't exist in python 2.7. We have enough version-specific #defines that we have no intention of having a single compiled numpy work against multiple python versions

[mattip]

Thanks Eric