Use BASE64_PAD macro instead of literal '=' in fast path

Replace hardcoded '=' characters with the BASE64_PAD macro
for consistency with the rest of the codebase.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: gpshead

Modules/binascii.c

@@ -188,7 +188,8 @@ base64_decode_fast(const unsigned char *in, Py_ssize_t in_len,
          * approach; on modern pipelined CPUs this is faster than bitmask tricks
          * like XOR+SUB+AND for zero-detection which have data dependencies.
          */
-        if (inp[0] == '=' || inp[1] == '=' || inp[2] == '=' || inp[3] == '=') {
+        if (inp[0] == BASE64_PAD || inp[1] == BASE64_PAD ||
+            inp[2] == BASE64_PAD || inp[3] == BASE64_PAD) {
             break;
         }