[python] RFC: supporting python 2 and 3 with one codebase

[Justus Winter]

Hi everyone :)

attached is a patch series that makes the notmuch python bindings
compatible with both python 2.x and python 3.x.

There are some workarounds, but those are mostly in globals.py and in
my opinion the benefit of supporting both versions with one codebase
is totally worth it.

Patch seven adds missing unicode conversions that should be applied
even if the rest of the patchset is not.

Cheers,
Justus