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/Objects/stringlib/formatter.h b/Objects/stringlib/formatter.h
index 139b56c..be0d897 100644
--- a/Objects/stringlib/formatter.h
+++ b/Objects/stringlib/formatter.h
@@ -414,7 +414,7 @@
     STRINGLIB_CHAR *end = ptr + len;
     STRINGLIB_CHAR *remainder;
 
-    while (ptr<end && isdigit(*ptr))
+    while (ptr<end && Py_ISDIGIT(*ptr))
         ++ptr;
     remainder = ptr;