feat: allow lxml requirement in range of >=4,<6 (#523)

Updates the requirements on [lxml](https://github.com/lxml/lxml) to permit the latest version.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.0.0...lxml-5.0.0)

---
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>

pyproject.toml

@@ -72,7 +72,7 @@ py-serializable =  "^0.16"
 sortedcontainers = "^2.4.0"
 license-expression = "^30"
 jsonschema = { version = "^4.18", extras=['format'], optional=true }
-lxml = { version="^4", optional=true }
+lxml = { version=">=4,<6", optional=true }
 
 [tool.poetry.extras]
 validation = ["jsonschema", "lxml"]