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.
* Simply use "import opcode" to import the opcode module instead of tricks
using the imp module
* Use context manager for the output file
* Move code into a new main() function
* Replace assert with a regular if to check the number of arguments
* Import modules at top level
on compilers that support it (notably: gcc, SunPro, icc), the bytecode
evaluation loop is compiled with a new dispatch mechanism which gives
speedups of up to 20%, depending on the system, on various benchmarks.
