Bump lxml from 4.2.1 to 4.6.2

Bumps [lxml](https://github.com/lxml/lxml) from 4.2.1 to 4.6.2.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.2.1...lxml-4.6.2)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -4,7 +4,7 @@ certifi==2018.4.16
 chardet==3.0.4
 dataset==1.1.0
 idna==2.7
-lxml==4.2.1
+lxml==4.6.2
 Mako==1.0.7
 MarkupSafe==1.0
 normality==0.6.1