Update flask to 1.0.3 and drop python2 support
- Allow custom CLIs using FlaskGroup to set the debug flag without it always being overwritten based on environment variables.
- flask --version outputs Werkzeug's version and simplifies the Python version.
- A catch-all error handler registered for HTTPException will not handle RoutingException, which is used internally during routing.
- Passing the json argument to app.test_client does not push/pop an extra app context.
- :func:send_file encodes filenames as ASCII instead of Latin-1 (ISO-8859-1). This fixes compatibility with Gunicorn, which is stricter about header encodings than PEP 3333.