support showing subclassing deprecation warning only once
revert warning filter on deprecated class instantiation
only warn on direct subclasses of the deprecated class
split subclassing tests and default to warning only once per deprecated class
allow deprecation of a class with custom metaclass
allow overriding old and new class paths
allow deprecation of subclasses of already deprecated classes
do not test multiple instantation warnings
Merge pull request #519 from dangra/warn-once

Warn BaseSpider deprecation only once