python3: avoid using exceptions module
The `exceptions` module is no longer available in Python 3.x and
can be avoided easily. We just need to use a different way to evade
the name clash with the module's own ImportError. One way is to
use the `__builtins__` module that is available in both 2.x (including
2.4.3) and 3.x.
See also:
The exceptions are defined in the module exceptions. This module
never needs to be imported explicitly: the exceptions are provided
in the built-in namespace as well as the exceptions module.
(https://docs.python.org/2/library/exceptions.html)
Signed-off-by: Pavel Šimerda <pavlix@pavlix.net>