Allow lxml 5.0 and update poetry

What does the merge request do?

Closes #567 (closed) Closes #480 (closed)

The fix for #480 (closed) was added in lxml 5.0, but it's a minor fix, so I've only set lxml ">=4.5.0" and updated poetry to lxml==5.1.1.

Summary for release notes

Fix an issue where duplicate on text elements returned incorrect results.

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history