Flask's _request_ctx_stack is deprecated

[Randy Syring]

marsh/conftest.py:1: in <module>
    import keg
.tox/py39/lib/python3.9/site-packages/keg/__init__.py:6: in <module>
    from keg.app import Keg  # noqa: F401
.tox/py39/lib/python3.9/site-packages/keg/app.py:16: in <module>
    from keg.templating import _keg_default_template_ctx_processor, AssetsExtension
.tox/py39/lib/python3.9/site-packages/keg/templating.py:3: in <module>
    from flask.globals import _request_ctx_stack
.tox/py39/lib/python3.9/site-packages/flask/globals.py:100: in __getattr__
    warnings.warn(
DeprecationWarning: '_request_ctx_stack' is deprecated and will be remoevd in Flask 2.3.

[Sean Kooyman]

See pallets/flask#4750