Python 3 compatibility

[ChielBruin]

The current version of pymtp only works when using Python2. This as the has_key() function of dictionaries is removed in Python3 (see this). Replacing the two occurrences of this construct by in makes the module work* in both Python2 and Python3.

This same fix can also be found in py3mtp [link]

*It compiles and runs, but behaviour is slightly different due to changes in ctypes

[ChielBruin]

I also had some troubles packaging the module in its current state on master. Using from .main import * in __init__.py and prefixing the period for the imports in the other files fixed the problems I had.
It seems that the developer of py3mtp had the same issues, as he moved all the code to a single file. This is another solution to combat the same problems I encountered.

[eighthave]

Great that you are maintaining pymtp! I haven't touched this since 2013. How about you take over the maintenance? I can give you access to pypi to upload the releases there.

[ChielBruin]

Sounds good to me

[eighthave]

@nick125 what do you think about handing over PyMTP maintenance to @ChielBruin?

[ChielBruin]

It seems like @nick125 is not very active on GitHub anymore. His last public commit is from a couple years ago.