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)

* 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)

Fixes #1911

Fixes #1911

Fix handling of "no-cpp" test tag for error tests, where "cpp" is already removed from the list of test backends.

Closes #3575

Fix handling of "no-cpp" test tag for error tests, where "cpp" is already removed from the list of test backends.

Closes #3575

Closes https://github.com/cython/cython/issues/3632

This avoids the METH_O function signature by default, since it does not match normal Python semantics.

* Fix unicode name handling of no-args functions when allowing keywords.

* Fix a crash when a keyword argument is passed to a function that does not allow them. Previously, the reported key name was not set and thus NULL.

* Extend "always_allow_keywords" test to cover some edge cases.
Some are commented out as they currently only work with the fastcall implementation.