Closes https://github.com/cython/cython/issues/3430

This allows things to work like:
    # in pxd file
    from libc.math cimport sin

    # in py file
    if not cython.compiled:
        from math import sin  # previously failed with cython compile
           # error because it was assigning to a cdef name

This seems worthwhile because it makes it easier to write code
that re-assigns cdef names and so works in both modes.

Prevent calling PyObject_GC_Track() in __Pyx_CyFunction_New() when instantiating a fused function before the object fields of the fused function subtype are fully initialised.
See https://bugs.python.org/issue38392
Closes GH-3215.
Supersedes GH-3216.

Sprinkle lots of branch prediction hints in places where exceptions are being handled or otherwise exceptional cases occur.

Mainly helps for adding attributes to classes, but undoubtedly fixes other things too.

* Add is_partitioned and partition to libcpp.algorithm
* Add is_sorted and is_sorted_until to libcpp.algorithm
* Add partition_copy, stable_partition, and partition_point to libcpp.algorithm
* Move tests for sort and partial_sort into cpp_stl_algo_sorting_ops, add greater to libcpp.functional
* Add partial_sort_copy to libcpp.algorithm
* Add stable_sort to libcpp.algorithm
* Add nth_element to libcpp.algorithm
* Add missing except specifiers in libcpp.algorithm

Fix refleak in limited-api case for cross-module shared extension types and store them correctly as PyTypeObject* instead of a generic (and unusable) PyObject*.

The current wording implies that users can expect it to work, which isn't really the current state.
It seems better not to claim too much until it's a bit more advanced.

Prevent calling PyObject_GC_Track() in __Pyx_CyFunction_New() when instantiating a fused function before the object fields of the fused function subtype are fully initialised.
See https://bugs.python.org/issue38392
Closes GH-3215.
Supersedes GH-3216.

Assume that code passed into cython.inline() is always NumPy 1.7+ clean, so that we can get rid of the C compiler warning about deprecated NumPy API usage.

Revert: Downgrade XCode on travis for Py2 to see if that fixes the clang build problems.

Try to resolve clang++ compilation issues on travis/macos. Apparently, CXXFLAGS are not being looked at.

Add __Pyx_IS_TYPE() macro to mimic the new "Py_IS_TYPE()" macro in CPython 3.9 that replaces the "Py_TYPE() == …" type check pattern.

Disable "unicode_imports" test in Py2 because it's too fiddly to get working on Windows/appveyor, also because Py2 distutils requires module names to be byte strings.