Split a combined assert in two to avoid an unused C temp variable when assertions are disabled. (GH-3870)

`xml.etree.ElementTree.Element.getiterator()` was deprecated in Python
2.7 & 3.2 and removed in Python 3.9.

Add "check_size ignore" to all NumPy.pxd class declarations to silence the useless size warnings about them.
See https://github.com/numpy/numpy/pull/432#issuecomment-689846159
Also remove the useless "extern" modifiers from cdef classes declared in "extern" sections.

Fixes GH-2749.

Rename "GCC_DIAGNOSTIC" macro to make it Cython specific and make it available to other utility code sections.

Prevent compile error when the result of repr() is assigned to a "unicode" LHS with language_level=3.
Closes https://github.com/cython/cython/issues/3736

Using Py_UNICODE to store lone surrogates makes Py3 join surrogate pairs on 16-bit Unicode platforms (Windows) when reading them back in, although we correctly processed them before.
Instead, we now use the "unicode_escape" codec to store byte strings, because it can return surrogate characters (which the other codecs cannot).

Fixes https://github.com/cython/cython/issues/2093

Disable Py_UNICODE fallback for __Pyx_UnicodeContainsUCS4() in Py3.9 since Py_UNICODE is deprecated and PEP-393 unicode is practically required.

* Really only use PyUnicode_FromUnicode() for strings that contain lone surrogate, not for normal non-BMP strings and not for surrogate pairs on 16bit Unicode platforms.

See https://github.com/cython/cython/issues/3678

* Extend buildenv test to debug a MacOS problem.
* Add a test for surrogate pairs in Unicode strings.
* Limit PyUnicode_FromUnicode() usage to strings containing lone surrogates.
* Accept ambiguity of surrogate pairs in Unicode string literals when generated on 16bit Py2 systems.

See https://github.com/cython/cython/pull/3677

* Validate and fix temp releasing (GH-3708)
Backports 92147baf.

    * Fix a temp leak in the type init code.
    * Fix temp leaks in fused types initialisation code.
    * Correctly release the buffer index temps allocated for index calculations.
    * Make tests fails hard if a temp variable is not released at the end of a generated function.
    * Fix temp leak in switch statement code.
    * Make end-to-end tests fail on refnanny output.
    * Fix result temp leak in PyTypeTestNode.
    * Fix result temp leak in external type/function import code and enable the refnanny check for them.
    * Fix temp leak when try-return-finally is used in generators.
    * Make it explicit when an allocated temp is not meant to be reused.
    * Fix temp leak when assigning to the real/imag attributes of complex numbers.
    * Fix temp leak when assigning to a memoryview slice.
    * Clean up "num_threads" result temp in parallel section, not only in prange loop.
    * Fix temp leak in Pythran buffer setitem code.
    * Simplify NumPyMethodCallNode since it does not need the Python function anymore. Previously, it generated code that needlessly looked up the Python function without actually using it.
    * Fix temp leak when deleting C++ objects.
    * Add a test that actually reusing temps when deleting C++ objects works correctly.

Fixes #3712 (hopefully) by reverting a small part of bbef4d74

exec() did not allow recent Python syntax features in Py3.8+ due to https://bugs.python.org/issue35975
Closes https://github.com/cython/cython/issues/3695

Rebased 1bb26b96 for cython 0.29.x,
and made conditional to Python 3.  This does not solve the original staticmethod problem
in Python 2 but it does resolve classmethod problems in Python 3.9.  Therefore, it unbreaks
other packages on Python 3.9, improves overall state for Python 3 and does not introduce
regressions in Python 2.