Remove incorrect dict unpacking optimisation that leaked external dict changes into the result (GH-4091)

Closes https://github.com/cython/cython/issues/3227

Creating global CDEFS for holding compiler definitions. Then pass it to the distutils.

Also add .vscode folder to .gitignore

Fixes https://github.com/cython/cython/issues/4096

Fix optimised_pow2_inplace() doctest on Python 3.10 because the error message changed.

Python 3.9 error message:
    unsupported operand type(s) for ** or pow(): 'int' and 'str'

Python 3.10 error message:
    unsupported operand type(s) for **=: 'int' and 'str'

Re-disable third-party integration testing in Py2.6 entirely since most packages aren't compatible anymore (and I don't want to dig up the versions that are).

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.