pep8 fixes
more pep8 fixes, removed duplicate SNI check for nsupdate.info

the super() call would do a duplicate check, so remove the nsupdate.info
check / SNI warning from base class.

in general, testing this based on .startswith("https://nsupdate.info") might
not work as we also have ipv4.nsupdate.info and ipv6.nsupdate.info.
Also, SNI might be used on other sites, too.
remove redundant parentheses, simplify boolean return

(except for the return tuples, where you likely put them intentionally for better visibility of the tuple)
add comment about reference-before-assignment bug
Merge pull request #11 from nsupdate-info/develop

pep8 fixes