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.

PyNumber_MatrixMultiply() and PyNumber_InPlaceMatrixMultiply() were
missing from number.pxd.

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

* Fixed issue where fused functions didn't register as cyfunctions

Issue https://github.com/cython/cython/issues/3384

This happened where Cython was using a shared API because the
CyFunction type imported from the shared api wasn't the same as
used to initialize the fused function tp_base.

Default arguments are now copied when the bound fused function is created.

Closes #3331.

Closes #1427 

* omp declare privates on outer loop, since inner loops are not 'omp parallel for'ified

They'd generate two calls - one exception checked and one not

Closes #3330.

The pickle functions __reduce__ and __setstate_cython__ were being
visited in the module scope (where they were undeclared) rather
than the class scope where they were declared

Make sure we always use the correct post-analysed bases, mkw and metaclass in Python class creation by making the PyClassDefNode own it and other nodes only refer to it, rather than risking stale references in subnodes.
(Alternatively, ownership could be distributed across the subnodes, but it's a bit unclear how, and how to refer to the other subnodes then.)
Closes #3338.

Closes #3337.

Add warning that no Cython-specific tests should be added to the PEP-492 coroutine test file copied from CPython.

Closes GH-3238

Remove legacy imports from IPython integration since the "py3compat" module changed in IPython 7.11. (GH-3297)

Import "Iterable" ABC class from "collections.abc" in Py3 instead of deprecated "collections" package.