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
]
azure = [
    'azure-batch>=8.0.0',
    'azure-cosmos>=3.0.1,<4',
    'azure-datalake-store>=0.0.45',
    'azure-identity>=1.3.1',
    'azure-keyvault>=4.1.0',
    'azure-kusto-data>=0.0.43,<0.1',
    'azure-mgmt-containerinstance>=1.5.0,<2.0',
    'azure-mgmt-datalake-store>=0.5.0',
    'azure-mgmt-resource>=2.2.0',
    'azure-storage>=0.34.0, <0.37.0',
    'azure-storage-blob<12.0',
]
cassandra = [
    'cassandra-driver>=3.13.0,<3.21.0',
]
celery = [
    'celery~=4.4.2',
    'flower>=0.7.3, <1.0',
    'vine~=1.3',  
]
cgroups = [
    'cgroupspy>=0.1.4',
]
cloudant = [
    'cloudant>=2.0',
]
dask = [
    'cloudpickle>=1.4.1, <1.5.0',