Properly handle chained exceptions in Python 3

[nicoddemus]

Fix #215

[coveralls]

Coverage increased (+0.003%) to 99.252% when pulling 06eda3b on nicoddemus:chained-exceptions into 3bf8af2 on pytest-dev:master.

[The-Compiler]

Why not have this import at the top of the file? Might also want to do except ImportError: instead so you don't need sys if you do it at the top.

[nicoddemus]

Why not have this import at the top of the file?

I put it here locally mostly because this is function is rarely used. If you prefer I can move it to the top though, no problem.

Might also want to do except ImportError: instead so you don't need sys if you do it at the top.

In general I tend to avoid that because it might lead to hard to diagnose errors (see pytest-dev/pytest-mock#68).

[nicoddemus]

@The-Compiler gentle ping. 😁

[The-Compiler]

Fair point - seems fine to keep it here.

[nicoddemus]

Thanks!