Fixes GH-3534.

Fixes #3531.

* Fixed indexing temps for non-python objects.
* Moved cleanup into release_temps since the temps survived into the result_code.

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

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.

Closes https://github.com/cython/cython/issues/3419

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

Closes #2514.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/gen.html).

PySlice_Unpack() and PySlice_AdjustIndices() were missing from
slice.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/slice.html).

Closes GH-3418.

* Work around a C compiler warning (in PyPy's cpyext).
* Avoid a C compiler warning in PyPy3.
* Fix a C compiler warning about an unused variable when compiling in PyPy.