fix(requirements-asynqp): Update flask to latest stable

[Ferenc-]

• This issue is described in
  ImportError: cannot import name 'soft_unicode' from 'markupsafe' (/usr/local/lib/python3.8/site-packages/markupsafe/__init__.py) pallets/markupsafe#284

• The recommended action is to upgrade Jinja2,
  ImportError: cannot import name 'soft_unicode' from 'markupsafe' (/usr/local/lib/python3.8/site-packages/markupsafe/__init__.py) pallets/markupsafe#284 (comment)
  but that only works with newer flask, so hence the flask upgrade.

• The exact backtrace in our case, that we are trying to avoid here is this:

Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/clients/test_asynqp.py:16: in <module>
    import tests.apps.flask_app
tests/apps/flask_app/__init__.py:5: in <module>
    from .app import flask_server as server
tests/apps/flask_app/app.py:10: in <module>
    from flask import jsonify, Response
venv/lib/python3.7/site-packages/flask/__init__.py:14: in <module>
    from jinja2 import escape
venv/lib/python3.7/site-packages/jinja2/__init__.py:12: in <module>
    from .environment import Environment
venv/lib/python3.7/site-packages/jinja2/environment.py:25: in <module>
    from .defaults import BLOCK_END_STRING
venv/lib/python3.7/site-packages/jinja2/defaults.py:3: in <module>
    from .filters import FILTERS as DEFAULT_FILTERS  # noqa: F401
venv/lib/python3.7/site-packages/jinja2/filters.py:13: in <module>
    from markupsafe import soft_unicode
E   ImportError: cannot import name 'soft_unicode' from 'markupsafe' (/home/circleci/repo/venv/lib/python3.7/site-packages/markupsafe/__init__.py)