Upgrade Flask to 2.0.0

[krivard]

I think we finally fell off the itsdangerous deprecation cliff. As late as yesterday we were seeing these warnings during unit testing:

  /usr/local/lib/python3.8/site-packages/flask/json/__init__.py:31: DeprecationWarning: Importing 'itsdangerous.json' is deprecated and will be removed in ItsDangerous 2.1. Use Python's 'json' module instead.

Today they flipped to ImportErrors (example):

_ ERROR collecting repos/delphi/delphi-epidata/tests/server/test_exceptions.py _
ImportError while importing test module '/usr/src/app/repos/delphi/delphi-epidata/tests/server/test_exceptions.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
repos/delphi/delphi-epidata/tests/server/test_exceptions.py:[7](https://github.com/cmu-delphi/delphi-epidata/runs/5250352604?check_suite_focus=true#step:15:7): in <module>
    from delphi.epidata.server._common import app
delphi/epidata/server/_common.py:3: in <module>
    from flask import Flask, g, request
/usr/local/lib/python3.[8](https://github.com/cmu-delphi/delphi-epidata/runs/5250352604?check_suite_focus=true#step:15:8)/site-packages/flask/__init__.py:1[9](https://github.com/cmu-delphi/delphi-epidata/runs/5250352604?check_suite_focus=true#step:15:9): in <module>
    from . import json
/usr/local/lib/python3.8/site-packages/flask/json/__init__.py:[15](https://github.com/cmu-delphi/delphi-epidata/runs/5250352604?check_suite_focus=true#step:15:15): in <module>
    from itsdangerous import json as _json
E   ImportError: cannot import name 'json' from 'itsdangerous' (/usr/local/lib/python3.8/site-packages/itsdangerous/__init__.py)

[dshemetov]

Flask 2.0.0 (commit 2f0c62f) seems to be the earliest version where this is fixed. We'd have to understand and test the backwards incompatibilities.

[krivard]

Hmmm, that sounds like a reason to wait until epidata v4 is merged

[melange396]

already done! currently up to Flask 2.2.5 as of #1175