Bump lxml from 4.6.5 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.5 to 4.9.1.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](https://github.com/lxml/lxml/compare/lxml-4.6.5...lxml-4.9.1)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: direct:production
...

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

requirements.txt

@@ -57,7 +57,7 @@ jupyter-core==4.6.1
 kitchen==1.2.6
 kiwisolver==1.1.0
 lazy-object-proxy==1.4.3
-lxml==4.6.5
+lxml==4.9.1
 matplotlib==3.1.1
 mccabe==0.6.1
 mock==3.0.5
@@ -210,7 +210,7 @@ jupyter-core==4.6.1
 kitchen==1.2.6
 kiwisolver==1.1.0
 lazy-object-proxy==1.4.3
-lxml==4.6.5
+lxml==4.9.1
 matplotlib==3.1.1
 mccabe==0.6.1
 mock==3.0.5