These were previously getting missed. Added code to specialize them and tests to prove it.

Fixes https://github.com/cython/cython/issues/3142

Also fixes https://github.com/cython/cython/issues/3460 - (seems related enough to go in the same PR)

Closes https://github.com/cython/cython/issues/3419

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/marshal.html).

This removes the `setup_requires` reference that AIUI is deprecated, in
favour of the `pyproject.toml` file which is what is supposedly needed nowadays.

The following code:
```
# cython: infer_types=True
import cython

@cython.cclass
class Foo:
    a: cython.double
    b: cython.double
    c: cython.double

    def __init__(self, a: cython.double, b: cython.double ,c: cython.double):
        self.a = a
        self.b = b
        self.c = c

def bar():
    l = []
    l.append(Foo(10, 20, 30))

    v = cython.declare(Foo, l[0])
    r = v.a + v.b
    print( r )

    v2 = cython.cast(Foo, l[0]) #Faster - No __Pyx_TypeTest() call
    r = v2.b + v2.c
    print( r )

bar()
```
works fine when compiled and throws an exception when interpreted: `TypeError: __init__() missing 2 required positional arguments: 'b' and 'c'`

It could be fixed if we change implementations as shown in the patch.
Also, added more tests for the cases I'm trying to fix
NB: Removed execution of `test_declare(None)` to make sure that the new `none_declare()` test works instead. `test_declare(None)` doesn't throw exception in pure mode but does it in the native mode

Replacing `hasattr(t, '__call__')` to `callable(t)` in the master branch broke the implementation and the tests because the construction was used to detect typedefs. To fix that I got rid of this check completely and replaced it to exact checks which also simplified the code

Changed `declare` implementation when initializing arguments are not provided. Now it correctly works with typedefs of the user classes and also directly support arrays:
    ```
    >>> EmptyClassSyn = cython.typedef(EmptyClass)
    >>> cython.declare(EmptyClassSyn) is None
    True
    >>> cython.declare(cython.int[2]) is not None
    True
    ```
Added missed return statement to `index_type` which made the following assigment possible:
    ```
        a = cython.declare(cython.int[2])
        a[0] = 1
    ```

Now they make their own tiny cache of the relevant exceptions at module initialization so they can still access these without the GIL.

closes https://github.com/cython/cython/issues/3406

While the directives in the source files are still called `distutils`, the [Python upstream documentation](https://docs.python.org/3/library/distutils.html) does not
recommend using distutils anymore, and rather points at setuptools, so avoid confusing new users by providing confusing legacy usage examples.

Also, this corrects one example in which Extension is imported too late, and would cause annoying errors when running `setup.py`.

The idea being that struct-types like memoryviews
can generate their own reference counting code
using a common interface with Python objects.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

- CyFunction seems to lead to deeper stacks on recursion.
- Passing integers through call layers can end up creating new int objects instead of keeping the identical objects ('is' test fails)'.

Closes #2514.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/gen.html).

Closes #2514.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/gen.html).

PySlice_Unpack() and PySlice_AdjustIndices() were missing from
slice.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/slice.html).

PySlice_Unpack() and PySlice_AdjustIndices() were missing from
slice.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/slice.html).

Closes GH-3418.