What it prints is irrelevant to static checking.

Currently the typeshed stub for the code already exports RegexFlag, so that the following passes mypy but fails at runtime:
```
from re import *

def foo(flag: RegexFlag):
    return match("[a-z]+", "ABC", flag)

print(foo(IGNORECASE))
print(foo(VERBOSE))
```
I think it's good to add it to `__all__` so this code will not put the type checker to shame, and it would be good to document it.

One thing I discovered when developing this example: there doesn't seem to be a flag to represent 0 (zero), i.e. "no flags".  And foo(0) is a type error (even though it works fine at runtime).