Bump lxml from 3.8.0 to 4.2.3

[dependabot-preview]

Bumps lxml from 3.8.0 to 4.2.3.

Changelog

Sourced from lxml's changelog.

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.

• LP#1727864, GH#258: Add "-isysroot" linker option on MacOS as needed by XCode 9.

4.2.0 (2018-03-13)

Features added

• GH#255: SelectElement.value returns more standard-compliant and
  browser-like defaults for non-multi-selects. If no option is selected, the
  value of the first option is returned (instead of None). If multiple options
  are selected, the value of the last one is returned (instead of that of the
  first one). If no options are present (not standard-compliant)
  SelectElement.value still returns None.

... (truncated)

Commits

• 995a804 Prepare release of lxml-4.2.3.
• a4574fc Backed out changeset ff9850742604
• 11c14e0 Improve status messages during wheel build.
• c0e4e29 Include Py3.7 in appveyor build and exclude Py2.6.
• ca51f28 Run three parallel wheel builds instead of two.
• 065cd90 Prepare release of lxml 4.2.2.
• f37271f Minor doc cleanups.
• a788020 Update changelog.
• 9ecc7c5 Update readme.
• 2140507 Add a FAQ entry on namespaces.
• Additional commits viewable in compare view

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Superseded by #34.