Changed "declared after use" warning to error

* ENH: implement cpython/time.pxd
Co-authored-by: scoder <stefan_ml@behnel.de>

In 0.11.1 is was marked as an warning that would become an error.
3.0 is probably either the time to make it an error (or to remove
the note that it'll be an error in future)

Always use "new" string->float conversions

This ensures that 1_2_3 type numbers are handled correctly
whether or not the argument type is known.

Fixes https://github.com/cython/cython/issues/3958 (and thus the
currently broken builtin_float test)

Fix missing utility code for complex annotations

Generating utility-code for complex types is special-cased, but
it wasn't when they were generated in annotations.

Fixes https://github.com/cython/cython/issues/3949

Though pretty standard, I can see how the use of these terms could
reasonably be viewed as offensive.

MAINT: Add const qualifiers to char* types in pycapsule.pxd.

Eliminate C compiler warnings by making the constness of the
char* pointers in the declarations in pycapsule.pxd match
the actual types of the PyCapsule_* functions.

Use Py_TYPE() and __Pyx_SET_SIZE() functions

Replace direct access to PyObject members with functions like
Py_TYPE() and __Pyx_SET_SIZE().

Change suggested by Stepan Kasal on Cython-devel list

The C-API function applied in previous commit (https://github.com/cython/cython/commit/b37607f46b165e81f7af2b4eec2ae7f76f7717df) is now also obsolete.
This patch works for cython 0.29 and cpython after (https://github.com/python/cpython/commit/1e996c3a3b51e9c6f1f4cea8a6dbcf3bcb865060#diff-d2c90d06d281ca8dd7ac8b90109fc26755f7319dd209b5f9416efdecfa56e289).

Fix "__Pyx_PyObject_AsDouble()" special-casing to prevent de-optimisation through potential PyFloat unpack-repack cycles.

(Probably not as optimal since it can lead to de-optimisation when we actually need a PyFloat result.)

Add flags and missing exception declaration for PyOS_*() string conversion functions in cpython/conversion.pxd.