Use multi-line import statements

[kanzure]

This switches the syntax used for long import lists into a format that
uses multiple lines. One particular benefit of this format is that diffs
more clearly show when certain imported symbols were added or removed.

The original format looks like:

from submodule import x, y, z

And this new format looks like:

from submodule import (
    x,
    y,

    # comments!
    z,
)

[petertodd]

I don't think it's worth applying this format to the "from future" lines, as they're standardized things that aren't going to change much, if ever; adds clutter at the top of every file.

Applying it to the RPC and network message test modules seems reasonable though. How about you remove the from future changes and I'll merge the other changes.

[petertodd]

Went through the code re: imports and cleaned things up; new version is now v0.3.0 release.