* tests for other link extractors are moved to test_linkextractors_deprecated.py
* in Python 3 Link is converted to use native strings for urls
* minor cleanups
* rename LinkExtractorTestCase to BaseSgmlLinkExtractorTestCase
* add BaseLinkExtractorTestCase link extractor tests can inherit from
and decouple it from SgmlLinkExtractor
* add an extra check for deny_extensions
* xfail test_restrict_xpaths_with_html_entities for LxmlLinkExtractor explicitly