Py3's raise ... from ... syntax not supported in Py2

[edschofield]

On Python 3 (only), the raise ... from ... syntax can be used as follows:

class DatabaseError(Exception):
    pass

class FileDatabase:
    def __init__(self, filename):
        try:
            self.file = open(filename)
        except IOError as exc:
            raise DatabaseError('failed to open') from exc

(see PEP 3134). This ticket is a feature request for an equivalent mechanism to be provided as e.g. future.utils.raise_from().

[edschofield]

Added in v0.12.x maint branch.