I have no idea why these aren't flagged with python2.  I excluded
test-highlight.t for now to make this easier to review- the changed code is
committed to a repo, which has cascading changes on the rest of the test.

There's a mix of bytes and str in the imports dict of contrib/import-checker.py
that crashed it half way through listing out these errors.  I couldn't figure
out how to fix that properly, so I was lazy and applied this on py3, to find the
rest of the errors:

    diff --git a/contrib/import-checker.py b/contrib/import-checker.py
    --- a/contrib/import-checker.py
    +++ b/contrib/import-checker.py
    @@ -626,7 +626,12 @@ def find_cycles(imports):
         top.foo -> top.qux -> top.foo
         """
         cycles = set()
    -    for mod in sorted(imports.keys()):
    +    def sort(v):
    +        if isinstance(v, bytes):
    +            return v.decode('ascii')
    +        return v
    +
    +    for mod in sor...