Clean weakrefs before calling user's dealloc

Add PyGILState_Check to Includes/cpython/pystate.pxd

Closes #2926.

We must clean the weakreferences before calling the user's __dealloc__
because if the __dealloc__ releases the GIL, a weakref can be
dereferenced accessing the object in an inconsistent state or
resurrecting it.

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