Fix cpyext.longobject on win64

Merged Ondrej Baranovič requested to merge branch/py3.7-win64-cpyext-longobject into branch/py3.7

Fix the parameter and return types for longobject in cpyext on win64, fixing lib-python test_capi.test_long_and_overflow. Requires replacing sys.maxsize in cpyext.test.test_longobject with a corrected value on win64.

Also fix parameters in PyBuffer_FillInfo, since Signed doesn't make sense as a parameter type in cpyext. The only other occurrence of Signed I can see in cpyext function signatures are the hash functions, and it is OK there (CPython typedefs Py_hash_t as Py_ssize_t which is the same as Signed).

Running numpy.test('full') on this PR on win64 now returns only 8 fails/errors (most of which look related to PyPy's GC, not win64).

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