Parsel

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* ``Selector.remove_namespaces`` received a significant performance improvement
* The value of ``data`` within the printable representation of a selector
(``repr(selector)``) now ends in ``...`` when truncated, to make the
truncation obvious.
* Minor documentation improvements.

~~~~~~~~~~~~~~~~~~

* ``has-class`` XPath function handles newlines and other separators
in class names properly;
* fixed parsing of HTML documents with null bytes;
* documentation improvements;
* Python 3.7 tests are run on CI; other test improvements.

~~~~~~~~~~~~~~~~~~

* New ``Selector.attrib`` and ``SelectorList.attrib`` properties which make
it easier to get attributes of HTML elements.
* CSS selectors became faster: compilation results are cached
(LRU cache is used for ``css2xpath``), so there is
less overhead when the same CSS expression is used several times.
* ``.get()`` and ``.getall()`` selector methods are documented and recommended
over ``.extract_first()`` and ``.extract()``.
* Various documentation tweaks and improvements.

One more change is that ``.extract()`` and ``.extract_first()`` methods
are now implemented using ``.get()`` and ``.getall()``, not the other
way around, and instead of calling ``Selector.extract`` all other methods
now call ``Selector.get`` internally. It can be **backwards incompatible**
in case of custom Selector subclasses which override ``Selector.extract``
without doing the same for ``Selector.get``. If you have such Selector
subclass, make sure ``get`` method is also overridden. For example, this::

class MySelector(parsel.Selector):
def extract(self):
return super().extract() + " foo"

should be changed to this::

class MySelector(parsel.Selector):
def get(self):
return super().get() + " foo"
extract = get

~~~~~~~~~~~~~~~~~~

* ``Selector`` and ``SelectorList`` can't be pickled because
pickling/unpickling doesn't work for ``lxml.html.HtmlElement``;
parsel now raises TypeError explicitly instead of allowing pickle to
silently produce wrong output. This is technically backwards-incompatible
if you're using Python < 3.6.

~~~~~~~~~~~~~~~~~~

* Fix artifact uploads to pypi.

~~~~~~~~~~~~~~~~~~

* ``has-class`` XPath extension function;
* ``parsel.xpathfuncs.set_xpathfunc`` is a simplified way to register
XPath extensions;
* ``Selector.remove_namespaces`` now removes namespace declarations;
* Python 3.3 support is dropped;
* ``make htmlview`` command for easier Parsel docs development.
* CI: PyPy installation is fixed; parsel now runs tests for PyPy3 as well.