Issue #15767: Introduce ModuleNotFoundError, a subclass of ImportError. The exception is raised by import when a module could not be found. Technically this is defined as no viable loader could be found for the specified module. This includes ``from ... import`` statements so that the module usage is consistent for all situations where import couldn't find what was requested. This should allow for the common idiom of:: try: import something except ImportError: pass to be updated to using ModuleNotFoundError and not accidentally mask ImportError messages that should propagate (e.g. issues with a loader). This work was driven by the fact that the ``from ... import`` statement needed to be able to tell the difference between an ImportError that simply couldn't find a module (and thus silence the exception so that ceval can raise it) and an ImportError that represented an actual problem.

/* Cross platform case insensitive string compare functions
 */

#include "Python.h"

int
PyOS_mystrnicmp(const char *s1, const char *s2, Py_ssize_t size)
{
    if (size == 0)
        return 0;
    while ((--size > 0) &&
           (tolower((unsigned)*s1) == tolower((unsigned)*s2))) {
        if (!*s1++ || !*s2++)
            break;
    }
    return tolower((unsigned)*s1) - tolower((unsigned)*s2);
}

int
PyOS_mystricmp(const char *s1, const char *s2)
{
    while (*s1 && (tolower((unsigned)*s1++) == tolower((unsigned)*s2++))) {
        ;
    }
    return (tolower((unsigned)*s1) - tolower((unsigned)*s2));
}