For tests and doctests, it is often desirable to show or verify that a certain warning occurs. Unfortunately, if the warning has been hit previously, it will not be issued again. It would be great if there were a documented, unified way to reset this 'seen' flag.

Related stackoverflow posts showing that I'm not the first person to have this need:

* https://stackoverflow.com/questions/19428761/python-showing-once-warnings-again-resetting-all-warning-registries
* https://stackoverflow.com/questions/2418570/testing-warnings-with-doctest

Neither of these has a satisfactory solution.

The easy answer for unittest is to use assertWarns.  For versions where that doesn't exist you could implement the same technique yourself.  Exposing a 'clear everything' function in the warnings module might not be a bad idea, but otherwise for doctest I'd think the catch_warnings context manager would be the correct answer, and that is documented.

Given the above, I guess this issue is a request to expose a "clear everything" function in the warnings module, which I think is a reasonable idea.

(I think it would be nice to add some support stuff to doctest to make things like this easier, but that is a totally separate issue.)

I think that it's possible to get the desired behavior by setting a filter to "always".

```
    > cat warnme.py 
    import warnings

    for i in range(3):
        warnings.warn("oh noes!")

    > python warnme.py 
    warnme.py:4: UserWarning: oh noes!
    warnings.warn("oh noes!")
    
    > cat warnme2.py 
    import warnings

    warnings.simplefilter("always")
    for i in range(3):
        warnings.warn("oh noes!")

    > python warnme2.py 
    warnme2.py:5: UserWarning: oh noes!
    warnings.warn("oh noes!")
    warnme2.py:5: UserWarning: oh noes!
    warnings.warn("oh noes!")
    warnme2.py:5: UserWarning: oh noes!
    warnings.warn("oh noes!")
```

Does that meet the need?