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

* This issue is described in
  pallets/markupsafe#284

* The recommended action is to upgrade Jinja2,
  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)
```

tests/requirements-asynqp.txt

@@ -1,7 +1,7 @@
 aiohttp>=3.7.4
 asynqp>=0.6
-flask>=1.1.4,<2.0.0
+flask>=2.0.0,<3.0.0
 mock>=2.0.0
 nose>=1.0
 pytest>=4.6
-urllib3[secure]!=1.25.0,!=1.25.1,<1.27,>=1.21.1
+urllib3[secure]!=1.25.0,!=1.25.1,<1.27,>=1.21.1