py_limited_api calculates extension module filename incorrectly

[mithrandi]

In

setuptools/setuptools/command/build_ext.py

Lines 113 to 116 in 2a0db96

	if use_abi3:
	so_ext = get_config_var('SO')
	filename = filename[:-len(so_ext)]
	filename = filename + get_abi3_suffix()

a suffix of get_config_var("SO") is removed from the filename calculated by the superclass; however, the actual suffix that the superclass uses is get_config_var("EXT_SUFFIX"): https://github.com/python/cpython/blob/a9e7db8845551e5639e1fa8030a0482363501fa2/Lib/distutils/command/build_ext.py#L680. As these two values are the same on many platforms, this problem is often hidden, but where they are not the same (eg. CPython 3.3 on Linux), the generated extension filename is wrong and fails to import (it'll be something like foo.cpython-33m.abi3.so).

(Thanks to @dstufft for tracking down the exact misbehaviour causing this)