Closes https://github.com/cython/cython/issues/2770

Add missing import in doctest when the module is compiled (and StringIO has become a global cdef variable).

* Add a wrapper for posix <sys/uio.h>
* Add a script to generate tests for posix pxds
* Generate `tests/compile/posix_pxds.pyx`
* Update fcntl.pxd to POSIX.1-2017
* Update urls for posix/mman.pxd
* Update urls for posix/resource.pxd
* Add url for posix/select.pxd, and remove a `const`
* Update stat.pxd to POSIX.1-2017
* Update url for posix/wait.pxd

Closes https://github.com/cython/cython/issues/4522

Avoid pointing users to a link that just says "this section was moved to...". Just take them to the correct page first time.

This definition could be improved when variadic templates are supported at some point, but since the types are not really relevant for Cython it should be fine to use C variadics here (especially since I do not expect that variadic templates will be supported anytime soon).

Closes https://github.com/cython/cython/issues/2171

Closes https://github.com/cython/cython/issues/4313

Annotations attached to namenodes weren't correctly processed with "InterpretCompilerDirectivesTransform"

Closes https://github.com/cython/cython/issues/4514

Add a `visitchild()` helper method to VisitorTransform that correctly reassigns the result, which is easily forgotten.

For PyPy3.8 (the current release is 7.3.7)

* PyPy<7.3.8 declares a struct with the last fields tp_finalize, tp_print, tp_pypy_flags
* PyPy>=7.3.8 will declare a struct with the last fields tp_finalize, tp_vectorcall, tp_print

PyPy3.9 (not yet released) will declare a struct with the last fields tp_finalize, tp_vectorcall, tp_pypy_flags

See https://foss.heptapod.net/pypy/pypy/-/issues/3618

For PyPy3.8 (the current release is 7.3.7)

* PyPy<7.3.8 declares a struct with the last fields tp_finalize, tp_print, tp_pypy_flags
* PyPy>=7.3.8 will declare a struct with the last fields tp_finalize, tp_vectorcall, tp_print

PyPy3.9 (not yet released) will declare a struct with the last fields tp_finalize, tp_vectorcall, tp_pypy_flags

See https://foss.heptapod.net/pypy/pypy/-/issues/3618

Revert "Try to resolve build failures by switching back to CPython's distutils instead of using the one in setuptools 0.60."

This reverts commit 0ec33692.

Try to resolve build failures by switching back to CPython's distutils instead of using the one in setuptools 0.60.

See https://github.com/pypa/setuptools/pull/2896
See https://github.com/cython/cython/pull/4510

Use PyType_GetSlot() and friends in more places to reduce the special cases for type-specs / Limited-API (GH-4506)

Distutils uses threading internally.

Also silence some warnings about redefined classes and function signatures when setting up the builtin scope. This is at most a second-best solution since we may not notice legitimate bugs on our side this way. Better make sure we have good test coverage of builtins and related optimisations.

Closes https://github.com/cython/cython/issues/4503

This also solves a difficulty with the Cython import in setuptools' build_ext. We need to inherit from the one in distutils, so that setuptools can inherit from us. That leads to a circular dependency that goes either way depending on which gets imported first by users, and in what way (from-import or module import). This is built to match the code in

https://github.com/pypa/setuptools/blob/9f1822ee910df3df930a98ab99f66d18bb70659b/setuptools/command/build_ext.py#L14-L21

Closes https://github.com/cython/cython/issues/3541

Fix division-by-zero crash in tests when the C ompiler discards the computational body of a Cython coroutine.

Closes https://github.com/cython/cython/issues/3612

Fixes https://github.com/cython/cython/issues/1863

Also adds a check whether we have the GIL before doing so. This
is important because Py_UNBLOCK_THREADS is documented as unsafe
if we don't hold the GIL.

Closes https://github.com/cython/cython/issues/4137