Allow selecting specific doctests in test modules with "-k pattern", instead of always running all tests.

Closes https://github.com/cython/cython/issues/3674

Return statements inside of nested try-except statements lead to an incorrect control flow graph that did not take the outer 'finally' clauses into account.
Closes https://github.com/cython/cython/issues/3666

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

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

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

* 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

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

Closes #3575

Disable GC during hacked calls to PyType_Ready() that simulate a heap type for an actual non-heap type, because it can lead to crashes if GC visits such a hacked type.
Closes https://github.com/cython/cython/issues/3603

Also mark the case of an uninitialised memory view slice as "unlikely()" when refcounting it since it's most probably not a performance critical case.