Bump lxml from 4.1.1 to 4.6.3 in /docs

Bumps [lxml](https://github.com/lxml/lxml) from 4.1.1 to 4.6.3.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.1.1...lxml-4.6.3)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -35,7 +35,7 @@ jupyter-highlight-selected-word==0.1.0
 jupyter-latex-envs==1.3.8.4
 jupyter-nbextensions-configurator==0.2.8
 lazy-object-proxy==1.3.1
-lxml==4.1.1
+lxml==4.6.3
 MarkupSafe==1.0
 matplotlib==2.0.2
 matplotlib-venn==0.11.5