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
'Environment :: Web Environment',
            'Intended Audience :: Developers',
            'Intended Audience :: System Administrators',
            'License :: OSI Approved :: Apache Software License',
            'Programming Language :: Python :: 3.6',
            'Programming Language :: Python :: 3.7',
            'Programming Language :: Python :: 3.8',
            'Topic :: System :: Monitoring',
        ],
        author='Apache Software Foundation',
        author_email='dev@airflow.apache.org',
        url='http://airflow.apache.org/',
        download_url=(
            'https://archive.apache.org/dist/airflow/' + version),
        cmdclass={
            'extra_clean': CleanCommand,
            'compile_assets': CompileAssets,
            'list_extras': ListExtras,
        },
        test_suite='setup.airflow_test_suite',
        python_requires='~=3.6',
        project_urls={
            'Documentation': 'https://airflow.apache.org/docs/',
            'Bug Tracker': 'https://github.com/apache/airflow/issues',
            'Source Code': 'https://github.com/apache/airflow',
        },
    )
if __name__ == "__main__":
    do_setup()