[3.13] pythongh-143012: use Py_ssize_t cast for PyBytes_FromStringAndSize (pythonGH-143013) (python#143015)

pythongh-143012: use `Py_ssize_t` cast for `PyBytes_FromStringAndSize` (pythonGH-143013)
(cherry picked from commit 5989095)

Co-authored-by: AZero13 <gfunni234@gmail.com>

Author: miss-islington

Modules/posixmodule.c

@@ -1698,7 +1698,7 @@ convertenviron(void)
 #ifdef MS_WINDOWS
         k = PyUnicode_FromWideChar(*e, (Py_ssize_t)(p-*e));
 #else
-        k = PyBytes_FromStringAndSize(*e, (int)(p-*e));
+        k = PyBytes_FromStringAndSize(*e, (Py_ssize_t)(p-*e));
 #endif
         if (k == NULL) {
             Py_DECREF(d);