[issue31369] re.RegexFlag is not included in __all__, makes type inference less useful

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

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).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31369>
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