Update Flask dependency to remove a deprecation warning

[ibnesayeed]

As noted in #937 (comment) the version of Flask we use (1.0.2) is causing a deprecation warning.

====================================================== test session starts =======================================================
platform linux -- Python 3.8.2, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /app
collected 12 items                                                                                                               

validate_test.py ............                                                                                              [100%]

======================================================== warnings summary ========================================================
/usr/local/lib/python3.8/site-packages/flask/sessions.py:14
  /usr/local/lib/python3.8/site-packages/flask/sessions.py:14: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working
    from collections import MutableMapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html
================================================= 12 passed, 1 warning in 1.49s ==================================================

[ibnesayeed]

Fixed in #958, ready for reviews.