Issue #17173: Remove uses of locale-dependent C functions (isalpha() etc.) in the interpreter.

I've left a couple of them in: zlib (third-party lib), getaddrinfo.c
(doesn't include Python.h, and probably obsolete), _sre.c (legitimate
use for the re.LOCALE flag).
diff --git a/Python/mystrtoul.c b/Python/mystrtoul.c
index 52502cb..8a54cbf 100644
--- a/Python/mystrtoul.c
+++ b/Python/mystrtoul.c
@@ -99,7 +99,7 @@
     register int ovlimit;       /* required digits to overflow */
 
     /* skip leading white space */
-    while (*str && isspace(Py_CHARMASK(*str)))
+    while (*str && Py_ISSPACE(Py_CHARMASK(*str)))
         ++str;
 
     /* check for leading 0b, 0o or 0x for auto-base or base 16 */
@@ -138,7 +138,7 @@
                 /* skip all zeroes... */
                 while (*str == '0')
                     ++str;
-                while (isspace(Py_CHARMASK(*str)))
+                while (Py_ISSPACE(Py_CHARMASK(*str)))
                     ++str;
                 if (ptr)
                     *ptr = str;
@@ -266,7 +266,7 @@
     unsigned long uresult;
     char sign;
 
-    while (*str && isspace(Py_CHARMASK(*str)))
+    while (*str && Py_ISSPACE(Py_CHARMASK(*str)))
         str++;
 
     sign = *str;