Cython support should use new_build_ext

[jaraco]

In #1229 and this comment, scoder recommendeds to prefer new_build_ext to build_ext.

[jaraco]

As observed in #1271, new_build_ext isn't viable without some transition support.

[jakirkham]

Maybe we get stuck in some recursive case here?

[jakirkham]

cc @scoder (for awareness)

[scoder]

The Cython project is currently updating new_build_ext here to match the older command:
cython/cython#4498