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.

When compiling pyarrow with cython 3.0 we get an error about the copy constructor of an object has been deleted on the generated context switch code.

Also make the "cpp_temp_assignment" test only run in C++11 since it is based on `std::move()`.

The condition wasn't triggering so code wasn't being tested (). This applies to:
```
def f(vector[double] x):
  pass
```
which should now generate:
```
__pyx_v_a = __pyx_convert_vector_from_py_double(values[0]); if (unlikely(PyErr_Occurred())) __PYX_ERR(0, 5, __pyx_L3_error)
...
__pyx_r = __pyx_pf_4cnvt_f(__pyx_self, __PYX_STD_MOVE_IF_SUPPORTED(__pyx_v_a));
```

I haven't included any specific tests because I think it should be covered by the existing test-suite (now that it works) and it's only a performance improvement - it doesn't allow any new behaviour.

See https://github.com/cython/cython/issues/4163

(Tried the "pattern" argument to loadTestsFromModule() in the unittest module, but it's not always taken into account, so manual selection is what works.)

Disable "cycoverage" test job since it currently fails, probably dude to issues with the "Cython.Coverage" plugin.

Add a coverage test job with a compiled Cython package to compare plain Python and compiled Cython both for completeness and speed.