add new check for multiple imports

[JasonMWhite]

This PR introduces a new import check, restricting imports to one import per line:

from foo import bar  # OK
from foo import baz # OK
from foo import bar, baz  # forbidden

This PR also ran up against other rule checks, so I created a new assertion checker that allows you to limit which codes are verified. This makes tests less fragile (no interdependencies between codes). Also, consumers may disable individual codes, so it's important that we test a code in isolation, whether or not it also fails other checks.

@cfournie @honkfestival

[cfournie]

This isn't strictly mentioned in the Google Python Style Guide; should this instead be included in a separate Shopify-specific checker?

[JasonMWhite]

I think it is: https://google.github.io/styleguide/pyguide.html?showone=Imports_formatting#Imports_formatting

[cfournie]

Specify args in the message if you want formatted output; currently it shows:

C:  8, 0: %s imports multiple items in one import statement (multiple-import-items)

Perhaps reword it to read:

Multiple items imported from %(module)s in one import statement.

And in add_message provide args={'module': node.modname}.

[cfournie]

Sorry, you're right, it is mentioned in the style guide.

[cfournie]

These don't commonly end in a period and are usually shorter, e.g.:

shopify_python/google_styleguide.py:192:0 I0011(locally-disabled) Locally disabling no-member (E1101)

If printed this would look like:

shopify_python/google_styleguide.py:192:0 C6010(multiple-import-items) Multiple items imported from os in one import statement. (C6010)

Maybe rephrase as:

Statement imports multiple items from '%(module)s'

or

Multiple items imported from '%(module)s'

[honkfestival]

Statement imports multiple items from '%(module)s'

👍

[cfournie]

You could expand upon this to explain the rationale.

[honkfestival]

👍

[honkfestival]

LGTM pending @cfournie's requested changes