Closes #3575

Closes https://github.com/cython/cython/issues/3632

This avoids the METH_O function signature by default, since it does not match normal Python semantics.

* Fix unicode name handling of no-args functions when allowing keywords.

* Fix a crash when a keyword argument is passed to a function that does not allow them. Previously, the reported key name was not set and thus NULL.

* Extend "always_allow_keywords" test to cover some edge cases.
Some are commented out as they currently only work with the fastcall implementation.

See https://github.com/numpy/numpy/pull/16266

Use a more efficient way to check for PyCFunction and CyFunction at the same time since checking for PyCFunction became more costly in Py3.9.

Use an inlined version of PyType_HasFeature() instead of directly accessing "tp_flags" in the generated code.

See https://github.com/cython/cython/issues/3625

The names of Cython's internal types (functions, generator, coroutine, etc.) are now prefixed with the shared module name, instead of making them look like homeless builtins.
See https://bugs.python.org/issue20204
See https://github.com/python/cpython/commit/490055a1673b524da2ebe2312f072aba2a826036

See https://github.com/cython/cython/pull/3616

When test_assert_path_exists or test_fail_if_path_exists
was used on a function containing a CompilerDirectivesNode
it was inherited by that CompilerDirectivesNode. Therefore
you got misleading test failures if the path was in the
function but not within that CompilerDirectivesNode.

* Fix #3588: Make existing overflow code safe

Signed overflow is undefined behaviour in C and compilers can and do optimized on that basis.

* Speed up our overflow impls

Note this is primarily based on performance of compilers which do not
support __builtin_add_overflow (i.e., not Clang >= 3.4 & gcc >= 5),
mostly looking at several gcc 4 releases (used by older, supported,
RHEL releases and Debian 8 "Jessie") and MSVC.

* Use __builtin_XXX_overflow if available

This is much quicker in general, as these all just then read the overflow flag from the status register.

Disable GC during hacked calls to PyType_Ready() that simulate a heap type for an actual non-heap type, because it can lead to crashes if GC visits such a hacked type.
Closes https://github.com/cython/cython/issues/3603

Disable GC during hacked calls to PyType_Ready() that simulate a heap type for an actual non-heap type, because it can lead to crashes if GC visits such a hacked type.
Closes https://github.com/cython/cython/issues/3603