Generate proper co_flags

[pfalcon]

Currently, dumping co_flags when comparing CPython vs python-compiler compilation results is disabled. Indeed, "async func" flag is definitely not handled, and probably other flags aren't dealt right with too.

[pfalcon]

Per https://github.com/pfalcon/python-compiler/issues/7#issuecomment-531453470, I switched working on my own bytecode compiler, written from scratch. I unlikely will work on this issue anytime soon, but leaving it open for possible interested parties.