Export warning classes and add PyErr::warn_explicit()

[birkenfeld]

Fixes #2741

[samuelcolvin]

LGTM, two comments:

• would it be clearer if we moved warnings to pyo3::warning?
• How hard would it be to add a method so we could call PyErr::warn_from_type::<PyUserWarning>(....) (like you can do dict.is_instance_of::<PyAny>)?

[birkenfeld]

1. there wouldn't be much in it, and since Warnings are Exceptions anyway...
2. possible, but I feel the combinatoric explosion of methods (what about a counterpart for warn_explicit...) is not worth the few times when it is needed.

Of course, these are my opinions, other maintainers may disagree.

[davidhewitt]

Thanks for this! I'm ok with pyo3::exceptions as a reasonable home for these for now, though I'm also not sure either way. It should be possible to re-home them in the future if we want.

As for the typed methods, I'm unconvinced we need them for now - users can always define wrappers e.g. user_warning(py, msg) to do exactly what you want.

[davidhewitt]

It looks like PyPyErr_WarnExplicit does exist as a symbol in PyPy shared lib, so I think we can update the ffi definition and enable warn_explicit on pypy.

[davidhewitt]

Perhaps worth expanding this doc to say it's equivalent to warnings.warn (and similar for warn_explicit)?

[davidhewitt]

Looks good to me, thanks!