Add isctest.check.ede() helper for pytest

Add isctest.check.ede() helper for pytest

Add a utility function to check for EDE options present in the DNS
message. The primary benefit of this helper function is that it
handles the compatibility issues with different dnspython versions
and the actual test code doesn't have to deal with that any more.

Rather than using the convenience .extended_errors() method
introduced in dnspython 2.7.0, iterate over the options and find
EDEOption types, which is supported from 2.2.0 onwards.

To work around the issue of using dns.edns.EDECode to specify EDE codes
in our tests, create an isctest.compat.EDECode wrapper. This can be used
even with dnspython versions prior to 2.2.0 and will simply result in
no-op, since EDE isn't supported in the older dnspython anyway.