Add various declarations for the CPython C API

Minor typo of the word "declare" in user guide

Closes GH-2811.

Fixes GH-2811.

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.

Remove Cython/Includes/Deprecated

Avoid generating a tp_new() function for extension types that do not need their own one.

See #1555.

In test runner, change "compiling test" back to "compiling and running test" when executing a runnable test, which was accidentally lost before.