Fixes https://github.com/cython/cython/issues/3642

Add "show_all_warnings" option to cythonize() to provide a simpler interface for (temporarily) setting "Errors.LEVEL".

See #3625.

Add a test that the default exception return value "-1" for integer return types also works for unsigned types.

Avoid "possible loss of data" warning in MSVC by adding redundant casts to code that gets discarded later.

Exclude "long long" overflow checking code from compilation when HAVE_LONG_LONG is not set by CPython.

classmethod() should fall back to calling PyClassMethod_New() instead of deciding for itself that the type is invalid. (GH-3660)

Closes #3659.

Fixes https://github.com/cython/cython/issues/3642

(Although that seems a bug – deep copying should probably just keep the control flow graph intact.)

Fix detection of reusable utility code: whenever we do string and/or unbound method replacements at all, it's not reusable.

Fixes https://github.com/cython/cython/issues/3647

At least one (in ModuleNode) is a real bug. The others are just
getting the code compatible with Cython again`

See https://github.com/cython/cython/issues/3646
Supersedes https://github.com/cython/cython/pull/3649

Revert "Disable "c_api_binop_methods" directive in 0.29.x and provide it only as an enabled forward option."

This reverts commit 4ac64ddf.

* Add __Pyx_SET_SIZE() function: use Py_SET_SIZE() on Python 3.9.0a4
and newer, or use Py_SIZE() as an l-value on older Python versions.

Py_SIZE() must not be used as an l-value anymore in Python 3.9:
Py_SET_SIZE() must be used instead:

* https://bugs.python.org/issue39573
* https://docs.python.org/dev/c-api/structures.html#c.Py_SET_SIZE

* Add __Pyx_SET_REFCNT() function: use Py_SET_REFCNT() on Python 3.9.0a4
and newer, or use Py_REFCNT() as an l-value on older Python versions.

Py_REFCNT() must not be used as an l-value anymore in Python 3.9:
Py_SET_REFCNT() must be used instead:

* https://bugs.python.org/issue39573
* https://docs.python.org/dev/c-api/structures.html#c.Py_SET_REFCNT

Use it in ModuleNode.generate_usr_dealloc_call():

* Replace ++Py_REFCNT(o) with __Pyx_SET_REFCNT(o, Py_REFCNT(o) + 1)
* Replace --Py_REFCNT(o) with __Pyx_SET_REFCNT(o, Py_REFCNT(o) - 1)