mib: remove a spurious indentation
version: prepare for release 0.7.0
main: provide an entry point to execute the module directly

The module can be executed with "python -m snimpy". This is the same
as using the entry point installed as `bin/snimpy`.
manager: use collections.MutableMapping instead of UserDict.DictMixin

The later is not available in Python 3.
py3: only use absolute import

Implied relative imports won't be supported with Python 3.
py3: use new format strings

While not strictly necessary, let's use new format strings. We try to
be compatible with both Python 2.6 (hence the indexes) and Python 3.