python: Remove reamining direct type comparisons.

python: Remove reamining direct type comparisons.

I've hit several bugs in this Python 3 work where the fix was some code
needed to be converted to use isinstance().  This has been primarily
around deadling with the changes to unicode handling.  Go ahead and
convert the rest of the direct type comparisons to use isinstance(), as
it could avoid a bug I haven't hit yet and it's more Pythonic, anyway.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>