Lower mbstring dependency
minor #10891 Lower mbstring dependency (nicolas-grekas)

This PR was merged into the 2.4 branch.

Discussion
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Lower mbstring dependency

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

Follow up of #10890

Commits
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6ab6713 Lower mbstring dependency