[BUG] std::string::npos not correctly mapped

[phoerious]

Describe the bug
libcpp.string.npos maps to std::npos, which doesn't exist and causes a compilation error:

error: ‘npos’ is not a member of ‘std’; did you mean ‘fpos’?

It should map to std::string::npos instead.

Sources:

• https://github.com/cython/cython/blob/master/Cython/Includes/libcpp/string.pxd#L8
• https://www.cplusplus.com/reference/string/string/npos/

Wrong:

cdef extern from "<string>" namespace "std" nogil:
    size_t npos = -1

Correct:

cdef extern from "<string>" namespace "std::string" nogil:
    const size_t npos

[0dminnimda]

This one was added 10 years ago, perhaps the author did not know about the presence of this in the standard library, or maybe they decided not to hope for an implementation in the standard library and make this constant explicitly -1.

It's just that if the implementation of the standard library uses a different value, the ported c/cpp code, in theory, may not work.
On the other hand, if someone directly compared npos to -1 (why?), Then the change could break existing code.

Anyway, PR is welcome

[phoerious]

Done.