Decide on a Python import style

[jdeanwallace]

As mentioned in a recent PR review, Jan and I both remember having a style rule where we prefer Python imports that look this:

import x.y
...
x.y.z()

Over this:

from x.y import z
...
z()

But we're not sure where this rule came from or where it was written. Either way, we should decide on an import style and stick to it moving forward.