py3: use format_exc instead of print_exception

[ghost]

Related: #1818

[Suor]

@MrOutis you left things like cause_tb here, no testing how from exc and implicit chaining works. Why did you close that? It was almost done.

[efiop]

Note that we no longer need cause_tb down below, so tb_list will only contain one entry, so we should get rid of it.

[ghost]

@MrOutis you left things like cause_tb here, no testing how from exc and implicit chaining works. Why did you close that? It was almost done.

@Suor, it was too big and it addressed changes that were not related to python 3. I'm going over it again.

[Suor]

Again, what about __context__?

[ghost]

@Suor , __context__? We are not using __context__, and that should be the matter of another PR, I guess 😕

[Suor]

Python 3 automatically assigns __context__ when you raise an exception while handling exception. So if you don't use either from exc or from None you still can show the original cause, this is usually an error in an error handler, which we probably want to see.

[ghost]

no testing how from exc and implicit chaining works

@Suor , implicit errors don't work with the current implementation.

[Suor]

@MrOutis implicit errors work when you raise exc, so if we are not showing them it means we broke it.

[efiop]

@Suor we didn't have __context__ earlier and didn't have implicit exceptions, nor do we use them in our code (3rd party libs might, be we haven't run into that yet). So no need to address those in this PR.

[efiop]

Thanks!