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
'salesforce': salesforce,
    'samba': samba,
    'segment': segment,
    'sendgrid': sendgrid,
    'sentry': sentry,
    'singularity': singularity,
    'slack': slack,
    'snowflake': snowflake,
    'spark': spark,
    'ssh': ssh,
    'statsd': statsd,
    'tableau': tableau,
    'vertica': vertica,
    'virtualenv': virtualenv,
    'webhdfs': webhdfs,  
    'winrm': winrm,  
    'yandexcloud': yandexcloud,
}
devel_all = list(set(devel +
                     [req for req_list in EXTRAS_REQUIREMENTS.values() for req in req_list] +
                     [req for req_list in PROVIDERS_REQUIREMENTS.values() for req in req_list]))
PACKAGES_EXCLUDED_FOR_ALL = []
if PY3:
    PACKAGES_EXCLUDED_FOR_ALL.extend([
        'snakebite',
    ])
PACKAGES_EXCLUDED_FOR_CI = [
    'apache-beam',
]
def is_package_excluded(package: str, exclusion_list: List[str]):