Bump mailman-hyperkitty from 1.1.0 to 1.2.0 in /core (#568)

Bumps [mailman-hyperkitty](https://gitlab.com/mailman/mailman-hyperkitty) from 1.1.0 to 1.2.0.
- [Release notes](https://gitlab.com/mailman/mailman-hyperkitty/tags)
- [Commits](https://gitlab.com/mailman/mailman-hyperkitty/compare/v1.1.0...1.2.0)

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

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core/requirements.txt

@@ -2,4 +2,4 @@
 # for version updates that isn't supported for contents inside the
 # Dockerfile.
 mailman==3.3.4
-mailman-hyperkitty==1.1.0
+mailman-hyperkitty==1.2.0