ImportError: cannot import name '_app_ctx_stack' from 'flask'

[jotelha]

CI test workflow fails with

 Traceback:
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_cli.py:4: in <module>
    import dtool_lookup_server
dtool_lookup_server/__init__.py:8: in <module>
    from flask_smorest import (
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/flask_smorest/__init__.py:5: in <module>
    from .spec import APISpecMixin
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/flask_smorest/spec/__init__.py:12: in <module>
    from flask_smorest.utils import prepare_response
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/flask_smorest/utils.py:6: in <module>
    from flask import _app_ctx_stack
E   ImportError: cannot import name '_app_ctx_stack' from 'flask' (/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/flask/__init__.py)

and this is likely due to upgrade to flask 3

Collecting flask (from dtool-lookup-server==0.18.0->-r requirements.txt (line 1))
  Downloading flask-3.0.0-py3-none-any.whl.metadata (3.6 kB)

according to several stack overflow posts, e.g. https://stackoverflow.com/questions/77240858/how-to-fix-cannot-import-name-request-ctx-stack-from-flask