Use multi-line import statements

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,
    )