This gives projects more time to move over, and reduces the number
of changes in the 0.25 series.

Fixes #1502

Closes #1496

Closes #1493

This partially reverts 65d3b977,
but is more correct.

Closes #1492

Implement numpy's import_array and friends in Cython

Add declarations for longintrepr.h

Revert "test: assert that isinf returns -1 for negative infinity"

This reverts commit 24cf7c96.

According to the numpy documentation npy_isinf merely provides an
equivalent of C99 isinf, which in turn is only required to return a
non-zero value for a given infinite argument.

The original commit breaks the test when compiled with clang & libc++.

Thanks to Martin Bammer for pointing this out.

check for build_ext object in import check

set IGNORE_EXCEPTION_DETAIL on MyException test

For case-insensitive systems.

Closes #1482

Test is failing on Python 3 because output is:

    builtin_type_inheritance_T608.MyException: 3

on py3, but

     MyException: 3

on py2

s/deprectated/deprecated/

When setuptools imports from Cython (Python 3.5),
setuptools.command.build_ext is defined,
but setuptools.command.build_ext has no attribute .build_ext, yet,
causing the `_build_ext.build_ext` to fail with an AttributeError.

By attempting to import the object, rather than its containing module, the check is more thorough.

This allows extending from it with a cdef class. Closes #724.