Bump mailman from 3.3.7 to 3.3.8 in /core (#592)

Bumps [mailman](https://gitlab.com/mailman/mailman) from 3.3.7 to 3.3.8.
- [Release notes](https://gitlab.com/mailman/mailman/tags)
- [Commits](https://gitlab.com/mailman/mailman/compare/3.3.7...3.3.8)

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

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

core/requirements.txt

@@ -1,5 +1,5 @@
 # This is a separate file from Dockerfile so that we can use dependabot
 # for version updates that isn't supported for contents inside the
 # Dockerfile.
-mailman==3.3.7
+mailman==3.3.8
 mailman-hyperkitty==1.2.0