Bump persistent from 6.0 to 6.1

Bumps [persistent](https://github.com/zopefoundation/persistent) from 6.0 to 6.1.
- [Changelog](https://github.com/zopefoundation/persistent/blob/master/CHANGES.rst)
- [Commits](zopefoundation/persistent@6.0...6.1)

---
updated-dependencies:
- dependency-name: persistent
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -264,7 +264,7 @@ papersize==1.3.0
     # via pdfimpose
 pdfimpose==1.1.0
     # via geotrek (setup.py)
-persistent==6.0
+persistent==6.1
     # via
     #   btrees
     #   zodb