With alpha2 without using the constraints https://github.com/apache/airflow/blob/constraints-master/constraints-3.6.txt , when accessing Webserver it fails with

```
Something bad has happened.
Please consider letting us know by creating a bug report using GitHub.

Python version: 3.6.12
Airflow version: 2.0.0a2
Node: 599861bd62ac
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1953, in full_dispatch_request
    return self.finalize_request(rv)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1970, in finalize_request
    response = self.process_response(response)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2269, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "/usr/local/lib/python3.6/site-packages/flask/sessions.py", line 387, in save_session
    samesite=samesite,
  File "/usr/local/lib/python3.6/site-packages/werkzeug/wrappers/base_response.py", line 479, in set_cookie
    samesite=samesite,
  File "/usr/local/lib/python3.6/site-packages/werkzeug/http.py", line 1217, in dump_cookie
    raise ValueError("SameSite must be 'Strict', 'Lax', or 'None'.")
ValueError: SameSite must be 'Strict', 'Lax', or 'None'.
```
setup.py
'cattrs~=1.0',
    'colorlog==4.0.2',
    'connexion[swagger-ui,flask]>=2.6.0,<3',
    'croniter>=0.3.17, <0.4',
    'cryptography>=0.9.3',
    'dill>=0.2.2, <0.4',
    'flask>=1.1.0, <2.0',
    'flask-appbuilder~=3.1.0',
    'flask-caching>=1.3.3, <2.0.0',
    'flask-login>=0.3, <0.5',
    'flask-swagger==0.2.13',
    'flask-wtf>=0.14.2, <0.15',
    'funcsigs>=1.0.0, <2.0.0',
    'graphviz>=0.12',
    'gunicorn>=19.5.0, <20.0',
    'iso8601>=0.1.12',
    'jinja2>=2.10.1, <2.12.0',
    'json-merge-patch==0.2',
    'jsonschema~=3.0',
    'lazy_object_proxy~=1.3',
    'lockfile>=0.12.2',
    'markdown>=2.5.2, <3.0',
    'markupsafe>=1.1.1, <2.0',
    'marshmallow-oneofschema>=2.0.1',
    'pandas>=0.17.1, <2.0',
    'pendulum~=2.0',
    'pep562~=1.0;python_version<"3.7"',
    'psutil>=4.2.0, <6.0.0',
    'pygments>=2.0.1, <3.0',
    'python-daemon>=2.1.1',