Log Message:
Update to 5.2.1. From the changelog:

* Fix #22 (pip package keeps upgrading all the time)
* Support PyPy
* Remove the HTTP Status 9001 test that caused unit test tracebacks
* Remove the completely-untested HTML tidy code
* Remove BeautifulSoup as a dependency
* Remove the XFN microformat parsing code
* Remove the rel_enclosure microformat parsing code
* Remove the rel_hcard microformat parsing code
* Remove the rel_tag microformat parsing code
* Replace the regex-based RFC 822 date parser with a procedural one
* Replace the Python-licensed W3DTF date parser
* Support HTML5 audio/source/video element relative URL's
* Remove the unparsed itunes_keywords key from the result dictionary
* Fix issue 321 just a little more (yet another code path was missed)
* Issue 62 (support georss and gml namespaces)
* Issue 296 (GUID's are always treated like relative URI's)
* Issue 334 (media:restriction element content is not returned)
* Issue 335 (sub-elements of media:group are not parsed and returned)
* Issue 342 (support multiple dc:creator elements)
* Issue 357 (loose parser breaks ampersands in link element URL's)
* Issue 374 (support the Podlove Simple Chapters namespace)
* Issue 380 (support media:rating element)
* Issue 384 (fix chardet support in Python 3)
* Issue 389 (elements in unknown uppercase namespaces are ignored)
* Issue 392 (tags element subverts 'tags' key in result dictionary)
* Issue 396 (Podlove Simple Chapters version 1.0 causes a KeyError)
* Issue 399 (docs call `request_headers` parameter `extra_headers`)
* Issue 401 (support additional dcterms and media namespaces elements)
* Issue 404 (support asctime datetime strings with timezone information)
* Issue 407 (decode forward slashes encoded as character entities)
* Issue 421 (delay chardet invocation as long as possible)
* Issue 422 (add return types docstrings)
* Issue 433 (update the list of allowed MathML elements and attributes)