This adds support for the newly-released Python 3.10. kgb itself had no

problems with this release that we could tell, but unit tests did fail

due to nose not being compatible with 3.10 (it uses

collections.Callable, which no longer exists).
We now use nose3 on Python 3. This is a drop-in replacement for nose

with modern Python 3 fixes.
Along with this, we're dropping Python 2.5, 2.6, 3.4, and 3.5 support.

We haven't been able to really test these in a while. Since technically

this is a breaking change, and we've moved to semantic versioning, we're

bumping kgb up to version 7.

Unit tests pass on all supported versions of Python (2.7 and 3.6-3.10).