Bump lxml from 4.1.1 to 4.2.1

[dependabot-preview]

Bumps lxml from 4.1.1 to 4.2.1.

Changelog

Sourced from lxml's changelog.

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.

• GH#261: The HTMLParser() now supports the huge_tree option.
  Patch by stranac.

Bugs fixed

• LP#1551797: Some XSLT messages were not captured by the transform error log.

• LP#1737825: Crash at shutdown after an interrupted iterparse run with XMLSchema
  validation.

Other changes

Commits

• 5653058 Prepare release of 4.2.1.
• 28a531b Be more conservative when not setting linker flags.
• bbcea38 Merge pull request #258 from frgtn/fix_macos_xcode_build
• e125690 Update changelog.
• 11623ab Update changelog.
• b398c51 LP#1755825: iterparse() failed to include the initial start event if the star...
• 14d8cc5 Improve documentation of the "tag" argument to the iter*() methods.
• 21dc579 Link directly to bug tracker from readme.
• 5dbbb2a Merge pull request #262 from dgreisen/patch-3
• 3e5025f add link to issue tracker in README.rst
• Additional commits viewable in compare view

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