Solution
click==8.1.3
Flask==1.0.2
itsdangerous==1.1.0
Jinja2==2.10.3
MarkupSafe==1.1.1
Werkzeug==2.1.2
> pip freeze
click==8.1.3
Flask==1.0.2
itsdangerous==1.1.0
Jinja2==2.10.3
MarkupSafe==1.1.1
Werkzeug==2.1.2
# make our virtual environment for testing
> python3 -m venv .venv
> source .venv/bin/activate
# install requirements
> pip install -r requirements.txt
# pinned versions are all installed
> flask run
* Environment: production
WARNING: This is a development server. Do not use it in a production deployment.
Use a production WSGI server instead.
* Debug mode: off
* Running on http://127.0.0.1:5000 (Press CTRL+C to quit)
# hit localhost:5000
> curl localhost:5000
Hello, World!
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