Update lxml to 4.6.4

[pyup-bot]

This PR updates lxml from 4.6.2 to 4.6.4.

Changelog

4.6.3

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

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

* A vulnerability (CVE-2021-28957) was discovered in the HTML Cleaner by Kevin Chung,
which allowed JavaScript to pass through.  The cleaner now removes the HTML5
``formaction`` attribute.

Links

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

[codecov]

Codecov Report

Merging #227 (6a57712) into master (a2f0d23) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #227   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files          27       27           
  Lines        1293     1293           
=======================================
  Hits         1268     1268           
  Misses         25       25           

[pyup-bot]

Closing this in favor of #230