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
"mongo": mongo,
    "mysql": mysql,
    "odbc": odbc,
    "openfaas": [],
    "opsgenie": [],
    "oracle": oracle,
    "pagerduty": pagerduty,
    "papermill": papermill,
    "plexus": plexus,
    "postgres": postgres,
    "presto": presto,
    "qubole": qds,
    "redis": redis,
    "salesforce": salesforce,
    "samba": samba,
    "segment": segment,
    "sftp": ssh,
    "singularity": singularity,
    "slack": slack,
    "snowflake": snowflake,
    "sqlite": [],
    "ssh": ssh,
    "vertica": vertica,
    "yandex": yandexcloud,
    "zendesk": zendesk,
}
EXTRAS_REQUIREMENTS: Dict[str, Iterable[str]] = {
    'all_dbs': all_dbs,
    'amazon': amazon,
    'apache.atlas': atlas,