py/parsenum: Simplify and generalise decoding of digit values.

This function should be able to parse integers with any value for the
base, because it is called by int('xxx', base).

py/parsenum.c

@@ -81,20 +81,18 @@ mp_obj_t mp_parse_num_integer(const char *restrict str_, size_t len, int base, m
     for (; str < top; str++) {
         // get next digit as a value
         mp_uint_t dig = *str;
-        if (unichar_isdigit(dig) && (int)dig - '0' < base) {
-            // 0-9 digit
-            dig = dig - '0';
-        } else if (base == 16) {
-            dig |= 0x20;
-            if ('a' <= dig && dig <= 'f') {
-                // a-f hex digit
-                dig = dig - 'a' + 10;
+        if ('0' <= dig && dig <= '9') {
+            dig -= '0';
+        } else {
+            dig |= 0x20; // make digit lower-case
+            if ('a' <= dig && dig <= 'z') {
+                dig -= 'a' - 10;
             } else {
                 // unknown character
                 break;
             }
-        } else {
-            // unknown character
+        }
+        if (dig >= base) {
             break;
         }