remove cython extension.

Now in cython package itself, as stable.
remove Cython Built-in extension notebook
remove more ref to cythonmagic
use Cython.load_ipython_ext to simplify
add whatsnew and uppercase C in load_ext

Use uppercase as this is what is in cython docs.
Make comm manager (mostly) independent of InteractiveShell

This makes it possible to use comms from wrapper kernels, without
instantiating the full IPython shell machinery.

The one remaining place where we need a reference to shell is to fire
pre_execute and post_execute hooks (which are needed to get mpl figures
right). This is a pure IPythonism, that it should be safe to ignore if
shell is not set.
Make comm_manager a property of kernel, not shell
Safeguard against comm instantiation with both session and kernel (corner case)
remove on_first_message authentication

in ZMQStreams. No need for weird, special first message.

- use regular cookie auth
- use url param for session id
Have /api return the IPython version

return version as a string, to be consistent with other APIs.

closes #6647
strip cell.trusted as a transient value

We write trust to cell.metadata.trusted now,
but used to write it to cell.trusted.
Just make sure we never get either from disk.
Merge pull request #6654 from Carreau/closes-6647

Have /api return the IPython version
removing unused line

Looks like a copy-paste error left an extra line around.
Merge pull request #6678 from perimosocordiae/patch-1

removing unused line
Fix version string comparison, capitalisation and typos.
Merge pull request #6676 from minrk/trust-save

strip cell.trusted as a transient value
Merge pull request #6510 from Carreau/removecython

Remove cython extension and examples.
Merge pull request #6629 from minrk/wscookie

remove on_first_message authentication
Merge pull request #6540 from takluyver/comms-mgr-needs-no-shell

Make comm manager (mostly) independent of InteractiveShell
Merge pull request #6221 from Carreau/cm4

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