It is (currently) Flask specific and hence cannot be shared in the sansio shared code.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry inCHANGES.rst summarizing the change and linking to the issue.
• Add.. versionchanged:: entries in any relevant code docs.
• Runpre-commit hooks and fix any issues.
• Runpytest andtox, no tests failed.