Freeze the test dependency packages for Python 2.x to prevent further deprecations. More and more packages drop Py2 support these days.

See https://github.com/cython/cython/issues/3919

The loop parameter of asyncio.sleep(), deprecated in Python 3.8, has
been removed in Python 3.10. asyncio.sleep() gets implicitly the loop
in Python 3.10 and it is safe to do the same on Python 3.6:
https://docs.python.org/dev/whatsnew/3.10.html#changes-in-the-python-api

Backport code from the master branch:

* commit 4e785d42
* commit a9fb7768

Backported from 3dc2b9df
Partial fix for https://github.com/cython/cython/issues/3919

The `test.support` has been split into sub-modules. Adapt `test_exceptions` (originally coped from CPython) for these changes.

Eliminate C compiler warnings by making the constness of the
char* pointers in the declarations in pycapsule.pxd match
the actual types of the PyCapsule_* functions.

Closes https://github.com/cython/cython/issues/4000

Warning message on cpdef variables (0.29.x)

Allowing these gives people the false impression that they
do something meaningful. "Fixes" https://github.com/cython/cython/issues/3959

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