Set language level in Cython's speed-up .pxd files since it no longer depends on the .py files that they correspond to.

Closes https://github.com/cython/cython/issues/2552

Keep AnnotationNode instead of the bare annotation expression in "entry.annotation" to get access to the string representation.

Do not include "u" string prefix in annotations since tools that process them probably expect Py3 string semantics anyway.

Prevent overflowing hash values for "Machines.Node" due to arbitrarily large unsigned pointer values.
Closes https://github.com/cython/cython/issues/3840

Specifically this allows public packed structs but may also allow other public declarations that require small amounts of utility code.

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.

Closes #3680.

Python's asyncio.coroutines uses an object to tag objects as coroutine functions. We now read this object and use it to tag Cython compiled coroutines as well.

It also includes tests to make sure `asyncio.iscoroutinefunction()` works as expected.
This doesn't fix `inspect.iscouroutinefunction()` (which uses a flag that can trigger undesirable behaviour for cython functions).

Closes https://github.com/cython/cython/issues/2273

Fixes GH-2749.

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

In C++ mode, structs can contain C++ classes. Therefore structs
should have their destructors called to ensure any class
contained is also destructed.

Also, a bit more thorough about ensuring constructor is generated where necessary.

Closes GH-3226.

Change test to use only integer calculations to prevent platform specific rounding issues, while keeping a reasonable distribution of integers across the PyLong digit ranges.

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

Fixes GH-2749.

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

Mainly by moving it to a separate section to make it easier
to find, however also added a small amount of extra information
about some of the obvious limitations.