Use __Pyx_SET_SIZE() and __Pyx_SET_REFCNT()

Don't access directly PyObject.ob_refcnt and PyVarObject.ob_size.

The C-API function applied in previous commit (https://github.com/cython/cython/commit/b37607f46b165e81f7af2b4eec2ae7f76f7717df) is now also obsolete.
This patch works for cython 0.29 and cpython after (https://github.com/python/cpython/commit/1e996c3a3b51e9c6f1f4cea8a6dbcf3bcb865060#diff-d2c90d06d281ca8dd7ac8b90109fc26755f7319dd209b5f9416efdecfa56e289).

Add flags and missing exception declaration for PyOS_*() string conversion functions in cpython/conversion.pxd.

Closes https://github.com/cython/cython/pull/3902

Disable the embedding test on MacOS-X to get the builds green again. Too many PRs depend on the travis tests to give a green light.

Also turn off the clang warning about deprecated declarations to try to reduce the size of the OSX logs.

The comments are taken from the C API documentation
(https://docs.python.org/3.9/c-api/cell.html).

Ensure that Cython exposes `PyBUF_MAX_NDIM` from Python as part of
`cpython.buffer` to allow access to developers.

Simplify the output stream capturing for the C compiler runs by using a temp file instead of threads.

I think this approach is more satisfactory than the old way it used to "work", where "fused_to_specific" was permanently added to the module scope containing the fused type (in this case the Cython scope), was applied in "Scope.lookup_type", but continued to have an effect on the scope forever.

Closes https://github.com/cython/cython/issues/3881

Closes https://github.com/cython/cython/issues/1355

Closes https://github.com/cython/cython/issues/3242

The comments are taken from the C API documentation: https://docs.python.org/3.9/c-api/codec.html

Closes https://github.com/cython/cython/issues/2251

Make type identifier escaping deterministic: hash() hashing lead to unpredictable random prefixes for long names across multiple Python runs.

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.