Bump lxml from 4.2.1 to 4.3.1

[dependabot-preview]

Bumps lxml from 4.2.1 to 4.3.1.

Changelog

Sourced from lxml's changelog.

4.3.1 (2019-02-08)

Bugs fixed

• LP#1814522: Crash when appending a child subtree that contains unsubstituted
  entity references.

Other changes

• Built with Cython 0.29.5.

4.3.0 (2019-01-04)

Features added

• The module lxml.sax is compiled using Cython in order to speed it up.

• GH#267: lxml.sax.ElementTreeProducer now preserves the namespace prefixes.
  If two prefixes point to the same URI, the first prefix in alphabetical order
  is used. Patch by Lennart Regebro.

• Updated ISO-Schematron implementation to 2013 version (now MIT licensed)
  and the corresponding schema to the 2016 version (with optional "properties").

Other changes

• GH#270, GH#271: Support for Python 2.6 and 3.3 was removed.
  Patch by hugovk.

• The minimum dependency versions were raised to libxml2 2.9.2 and libxslt 1.1.27,
  which were released in 2014 and 2012 respectively.

• Built with Cython 0.29.2.

4.2.6 (2019-01-02)

Bugs fixed

• LP#1799755: Fix a DeprecationWarning in Py3.7+.

... (truncated)

Commits

• 642a41b Prepare release of 4.3.1.
• 866e515 Remove Py3.7 from allowed build failures in travis.
• ee9dc10 Also set .doc field of attribute children (if any) during subtree migration.
• 3a8123d Replace obfuscated loop with a helper function that is called twice for two d...
• 3806d61 Disable a test under Windows that depends on library linking.
• 9a6db11 Rename appveyor script to more common name without leading dot.
• 10ee383 First build, then run the tests in appveyor.
• fc0a4d3 Run tests in appveyor.
• 201b712 LP#1814522: Fix a crash when appending a child subtree that contains unsubsti...
• 7303cad Prepare release of lxml 4.3.0.
• Additional commits viewable in compare view

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Superseded by #211.