Update opcode.h header comment to mention the source data file (GH-9935)

This is intended to help code explorers find out more about what's defined there.

Include/opcode.h

-/* Auto-generated by Tools/scripts/generate_opcode_h.py */
+/* Auto-generated by Tools/scripts/generate_opcode_h.py from Lib/opcode.py */
 #ifndef Py_OPCODE_H
 #define Py_OPCODE_H
 #ifdef __cplusplus

Tools/scripts/generate_opcode_h.py

@@ -3,7 +3,8 @@
 import sys
 import tokenize
 
-header = """/* Auto-generated by Tools/scripts/generate_opcode_h.py */
+header = """
+/* Auto-generated by Tools/scripts/generate_opcode_h.py from Lib/opcode.py */
 #ifndef Py_OPCODE_H
 #define Py_OPCODE_H
 #ifdef __cplusplus
@@ -12,7 +13,7 @@ extern "C" {
 
 
     /* Instruction opcodes for compiled code */
-"""
+""".lstrip()
 
 footer = """
 /* EXCEPT_HANDLER is a special, implicit block type which is created when