fixes `pyximport.install(pyimport=True)` on the darwin architecture

Add various declarations for the CPython C API

blocking all distutils from being compiled; removed distutils blacklists from the pyimport test and deleted the 'slow' test as it is now redundant

Minor typo of the word "declare" in user guide

Closes GH-2811.

Fixes GH-2811.

pyimport=True should work now on darwin without having to pre-import distutils directly or via pyxbuild

BUG: prefer sys.path before Cython/Include when searching for pxd files

Revert dictionary item iteration in "libcython.py" to use ".iteritems()" instead of ".items()" since the first is implemented explicitly by "libpython.py".

Avoid generating empty tp_new() functions

Avoid generating empty tp_new() functions if we can copy the function pointer from the parent at init time.
See GH-1555.