[py3]: Avoid changing dictionary while looping through it

[encukou]

This is continuation of #104:

• Do the same fix a few lines down
• Smoke-test the fix to the original issue by calling attribute_types() for our test data

[hroncok]

Will continue the discussion here:

@encukou wrote:

@tiran: Actually, the keys() is necessary. This is not a regular dict, but a python-ldap cidict (though it only holds OIDs). And cidict wasn't designed for just iterating over it.

I've just looked at this (out of curiosity) and this is what I found: cidict is an UserDict and that is iterabale on Python 3, but not on Python 2, the Python 2 documentation even says: For backward compatibility, instances of UserDict are not iterable.