Allow using warnings.warn() with a Warning

[eerovaher]

warnigns.warn() expects its first argument to be a str or a Warning (see also WarningMessage in typeshed) and using a different type can cause difficult to diagnose errors (as was reported in #10865):

>>> import warnings
>>> with warnings.catch_warnings():
...     warnings.filterwarnings("ignore", "test")
...     warnings.warn("Warning!")
...     warnings.warn(RuntimeWarning())
...     warnings.warn(0)
... 
<stdin>:3: UserWarning: Warning!
<stdin>:4: RuntimeWarning: 
Traceback (most recent call last):
  File "<stdin>", line 5, in <module>
TypeError: expected string or bytes-like object

Recently #11804 tried to help address this problem by making pytest.warns() check the type that was passed to warnings.warn(), but the checks are now too strict:

>>> import warnings
>>> import pytest
>>> with pytest.warns(Warning):
...     warnings.warn(RuntimeWarning(["warning", "context"]))
... 
Traceback (most recent call last):
    ...
    raise TypeError(
TypeError: Warning message must be str, got ['warning', 'context'] (type list)
>>> with pytest.warns(Warning):
...     warnings.warn(RuntimeWarning())
... 
Traceback (most recent call last):
    ...
IndexError: tuple index out of range

Both uses are explicitly permitted by Python. Furthermore, in the second case the IndexError is completely unexpected.

With my patch pytest.warns() allows warnings.warn() to be used with a Warning instance, except if a UserWarning is created by passing it arguments and the first argument is not a str. This is because if an invalid type was used in warnings.warn() then Python creates a UserWarning anyways and it becomes impossible for pytest to figure out if that was done automatically or not. However, even with that shortcoming pytest.warns() still behaves much better than it does right now.

Closes #11954

Here is a quick checklist that should be present in PRs.

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

[bluetech]

Thanks for the patch and the clear description.

I added a commit and rebase yours on top to make things a bit clearer, but the logic is the same except I removed the isinstance(wrn, Warning) check which I believe was unnecessary and made the typing not work out.