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.

Closes GH-3246.

This should complete #3171 in silencing -Wmissing-field-initializers.

* IPython magic: hide internal traceback when compilation fails
* IPython magic: hide internal traceback when build fails

This is more follow up to https://bugs.python.org/issue37250

The action taken is:
 - restore tp_print to not break all of the sdists on pypi for py38
 - remove tp_print for real in py39

In https://github.com/cython/cython/pull/3171 tp_print was initialized
for PY_VERSION_HEX >= 0x030800b4 however, when trying to use cython
with cpython master (aka py39) there are compile time exceptions due
to too many initializers:

   error: too many initializers for ‘PyTypeObject’ {aka ‘_typeobject’}

This fixes that by putting an upper bound on the ifdef of including
pt_print at the end of the object.

Make sure to include "Python.h" also from the "public" header file, which depends on its declarations.
Closes #3133.

Explicitly initialize tp_print in Python 3.8

When compiling cython-generated extension modules in Python3.8rc1
this error is emitted by the compiler:

  _ext.cpp:8104:1: error: missing initializer for member ‘_typeobject::tp_print’ [-Werror=missing-field-initializers]

The reason is that Python3.8 moved the tp_print slot (d917cfe4051) to
the end of the _typeobject struct and reused the original position for
tp_vectorcall_offset. The current generated code does not initialize the
deprecated tp_print slot that was moved to the end of the struct.

When compiling to C++ code and using automatic conversion from Python
str to C++ string, Cython inserts this function template but generates the warning:
warning: string.from_py:15:63: local variable 'length' might be referenced before assignment