runtests: Print shard number also for each command in the EndToEnd test error output to make it easy to find in the logs.

runtests: print the shard number in front of the test name to show which was the last test in case it hangs.

See https://github.com/cython/cython/pull/4583#issuecomment-1036452181

Fixes https://github.com/cython/cython/issues/4092

Nested cdef classes are not tested because they don't officially work
at this stage (see https://github.com/cython/cython/issues/4572).
It should be tested if/when they're fully supported.

Limited API: Fix line number reporting in tracebacks that accidentally used 'c_line' instead of 'py_line'. In fact, we cannot currently show the C line, so disable C line reporting completely here.

Follow PEP-484 and warn about "func(x: list = None)" without an explicit "typing.Optional[]" declaration.

See https://github.com/cython/cython/issues/3883

Avoid an unnecessary inefficiency in a regex, although it is not relevant in practice since the regex is only applied to Cython's only C code.
Found by GitHub CodeQL.

Rewrite the section that describes the differences between __cinit__() and __init__(), to make it clearer when to use which.

Partially reverts https://github.com/cython/cython/commit/1027db04fb7c5bbfea2b9bcdfce0b1f384e26a93

Make the extension types tutorial actually understandable by itself, without referring to obscure "previous code" and "our integration example" that come out of nowhere.

Allow globally forcing C file regeneration by setting the env var CYTHON_FORCE_REGEN=1, e.g. from external build systems.

Always regenerate .c/cpp output files when changing the Cython version. This is probably what users expect since silently keeping outdated files is error prone.

By disabling "CYTHON_FAST_THREAD_STATE" and "CYTHON_USE_EXC_INFO_STACK"

I think this still leaves some breakage in Coroutines.c but it's enough of a fix that Cython succeeds in building itself. Therefore I think it's worth doing now even if it doesn't fix everything.

Related to https://github.com/cython/cython/issues/4500