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
'distributed>=2.11.1, <2.20'
]
databricks = [
    'requests>=2.20.0, <3',
]
datadog = [
    'datadog>=0.14.0',
]
doc = [
    'sphinx>=2.1.2',
    'sphinx-argparse>=0.1.13',
    'sphinx-autoapi==1.0.0',
    'sphinx-copybutton',
    'sphinx-jinja~=1.1',
    'sphinx-rtd-theme>=0.1.6',
    'sphinxcontrib-httpdomain>=1.7.0',
    "sphinxcontrib-redoc>=1.6.0",
    "sphinxcontrib-spelling==5.2.1"
]
docker = [
    'docker~=3.0',
]
druid = [
    'pydruid>=0.4.1',
]
elasticsearch = [
    'elasticsearch>7, <7.6.0',
    'elasticsearch-dbapi==0.1.0',
    'elasticsearch-dsl>=5.0.0',
]