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.

Revert "Revert "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.""

This reverts commit bea8df39.

* Fixed crash on memoryview self-assignment
* Rename memoryview decref function to match what it does

https://github.com/numpy/numpy/pull/17150/files

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

Revert "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."

This reverts commit 1c947de7.

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

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

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

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/1355

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