Update lxml to 4.6.1

[pyup-bot]

This PR updates lxml from 4.5.2 to 4.6.1.

Changelog

4.6.1

==================

Bugs fixed
----------

* A vulnerability was discovered in the HTML Cleaner by Yaniv Nizry, which allowed
JavaScript to pass through.  The cleaner now removes more sneaky "style" content.

4.6.0

==================

Features added
--------------

* GH310: ``lxml.html.InputGetter`` supports ``__len__()`` to count the number of input fields.
Patch by Aidan Woolley.

* ``lxml.html.InputGetter`` has a new ``.items()`` method to ease processing all input fields.

* ``lxml.html.InputGetter.keys()`` now returns the field names in document order.

* GH-309: The API documentation is now generated using ``sphinx-apidoc``.
Patch by Chris Mayo.

Bugs fixed
----------

* LP1869455: C14N 2.0 serialisation failed for unprefixed attributes
when a default namespace was defined.

* ``TreeBuilder.close()`` raised ``AssertionError`` in some error cases where it
should have raised ``XMLSyntaxError``.  It now raises a combined exception to
keep up backwards compatibility, while switching to ``XMLSyntaxError`` as an
interface.

Links

• PyPI: https://pypi.org/project/lxml
• Changelog: https://pyup.io/changelogs/lxml/
• Homepage: https://lxml.de/

[codecov]

Codecov Report

Merging #298 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #298   +/-   ##
========================================
  Coverage    62.69%   62.69%           
========================================
  Files           80       80           
  Lines         8730     8730           
========================================
  Hits          5473     5473           
  Misses        3257     3257