Bump lxml from 4.6.5 to 4.9.1 (#429)

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](lxml/lxml@lxml-4.6.5...lxml-4.9.1)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -9,7 +9,7 @@ SQLAlchemy==1.3.8
 Flask-SQLAlchemy==2.4.0
 itsdangerous==1.1.0
 pyproj==2.6.1.post1
-lxml==4.6.5
+lxml==4.9.1
 OWSLib==0.20.0
 jsonschema==3.0.2 # downgrade from 3.2.0 on sept 29, 2020, issue 331, consider better fix
 openapi-spec-validator==0.2.8