Set _main_module when extending StockPickler

[numpand]

#23

[mmckerns]

I will accept this. Please make your test 3.x compatible.

try:
    from StringIO import StringIO
except ImportError:
    from io import BytesIO as StringIO

[numpand]

Just to be clear, do you want me to do another commit with the above change?

[mmckerns]

yes. you guys had mentioned possibly making a future pull request, so I wanted to also make sure that you were aware that dill is 3.x compatible. I appreciate the pull, and think it'd be cleaner if you made a second commit with the above change. Thanks, and I look forward to seeing what you guys have in mind. (Are you part of PISM like @citibob? That seems like an interesting project -- I have some frameworks that might apply well to PISM, see https://github.com/uqfoundation.)

[numpand]

I modified the test to be 3.x compatible (I did not test with Python 3.x, but hopefully the change is small enough that it should not be an issue). I am not part of PISM btw.

[mmckerns]

@numpand: Thanks, I appreciate the follow-up. I tested it against 3.x and the rest of the test suite, and it passes.