docs: Use the Cython + IPython lexers that come with Pygments to avoid having to maintain our own ones.

This broke C++ map iteration code, which needs a better fix.
See https://github.com/cython/cython/issues/4171
Reverts https://github.com/cython/cython/pull/4077

This reverts commit 415c0496.

Also remove ErrorType type defaults for nodes in ExprNodes

Closes https://github.com/cython/cython/issues/4155

Uses a special package prefix "cython.cimports", followed by the actual .pxd module/package names.
Also allows "import cython.cimports.libc.math as libc_math" etc., rather than only "from ... import ...".
All we need to guard against is that the import does not overwrite the name "cython".

https://foss.heptapod.net/pypy/pypy/-/issues/3404

Assume that any assignment to a variable that has a PEP-526 type annotation is the first assignment to that variable.
This makes literal list assignments to declared pointers work.

Fix pure Python struct test in Py<3.6 where keyword dicts are unordered. This means that cython.struct(x=int, y=float) has a non-deterministic order.

Closes https://github.com/cython/cython/pull/4183
Closes https://github.com/cython/cython/issues/4182

ALways mark the entry of generator expression functions as used. Because, why not, they probably are. And it avoids false warnings.
Fixes https://github.com/cython/cython/issues/1699

See https://github.com/cython/cython/issues/4177
See https://github.com/cython/cython/pull/4178

This is a common gotcha for new users who name their .pyx file after the C file that they want to wrap.
Closes https://github.com/cython/cython/issues/4177

Disable some plain Python (non Cython related) tests that fail in Py3.10 due to changed error messages.