makeopcodetargets.py: we need to import Lib/opcode.py

Issue #20021: use importlib.machinery to import Lib/opcode.py and not an opcode
module coming from somewhere else. makeopcodetargets.py is part of the Python
build process and it is run by an external Python program, not the built Python
program.

Patch written by Serhiy Storchaka.

Python/makeopcodetargets.py

@@ -3,14 +3,34 @@
 (for compilers supporting computed gotos or "labels-as-values", such as gcc).
 """
 
-import opcode
 import os
 import sys
 
 
+try:
+    from importlib.machinery import SourceFileLoader
+except ImportError:
+    import imp
+
+    def find_module(modname):
+        """Finds and returns a module in the local dist/checkout.
+        """
+        modpath = os.path.join(
+            os.path.dirname(os.path.dirname(__file__)), "Lib")
+        return imp.load_module(modname, *imp.find_module(modname, [modpath]))
+else:
+    def find_module(modname):
+        """Finds and returns a module in the local dist/checkout.
+        """
+        modpath = os.path.join(
+            os.path.dirname(os.path.dirname(__file__)), "Lib", modname + ".py")
+        return SourceFileLoader(modname, modpath).load_module()
+
+
 def write_contents(f):
     """Write C code contents to the target file object.
     """
+    opcode = find_module('opcode')
     targets = ['_unknown_opcode'] * 256
     for opname, op in opcode.opmap.items():
         targets[op] = "TARGET_%s" % opname