Bump lxml from 3.8.0 to 4.2.4

[dependabot-preview]

Bumps lxml from 3.8.0 to 4.2.4.

Changelog

Sourced from lxml's changelog.

4.2.4 (2018-08-03)

Features added

• GH#259: Allow using pkg-config for build configuration.
  Patch by Patrick Griffis.

Bugs fixed

• LP#1773749, GH#268: Crash when moving an element to another document with
  Element.insert().
  Patch by Alexander Weggerle.

4.2.3 (2018-06-27)

Bugs fixed

• Reverted GH#265: lxml links against zlib as a shared library again.

4.2.2 (2018-06-22)

Bugs fixed

• GH#266: Fix sporadic crash during GC when parse-time schema validation is used
  and the parser participates in a reference cycle.
  Original patch by Julien Greard.

• GH#265: lxml no longer links against zlib as a shared library, only on static builds.
  Patch by Nehal J Wani.

4.2.1 (2018-03-21)

Bugs fixed

• LP#1755825: iterwalk() failed to return the 'start' event for the initial
  element if a tag selector is used.

• LP#1756314: Failure to import 4.2.0 into PyPy due to a missing library symbol.

... (truncated)

Commits

• 5578c4d Update changelog.
• 2bc2bce Do not statically link compiled Python modules against libxml2 & friends.
• ab76824 Fix CFLAGS in travis build.
• 0974ac5 Include "*-rc" versions of libxml2/libxslt in parsed version when downloading...
• d062845 Prepare release of lxml 4.2.4
• 5184647 Support using pkg-config for xml2-config/xslt-config tools
• 9e3060d Include standard Py3.7 in travis builds.
• b24b0b7 Add a test for the fix in github ticket #268.
• 864a17e Fixing possible memory corruption if node is moved between docs
• 0a996c9 Fix output in wheel building script.
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #37.