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
'python-dateutil>=2.3, <3',
    'python-nvd3~=0.15.0',
    'python-slugify>=3.0.0,<5.0',
    'requests>=2.20.0, <3',
    'setproctitle>=1.1.8, <2',
    'sqlalchemy>=1.3.18, <2',
    'sqlalchemy_jsonfield~=0.9',
    'tabulate>=0.7.5, <0.9',
    'tenacity~=6.2.0',
    'termcolor>=1.1.0',
    'thrift>=0.9.2',
    'typing;python_version<"3.6"',
    'typing-extensions>=3.7.4;python_version<"3.8"',
    'tzlocal>=1.4,<2.0.0',
    'unicodecsv>=0.14.1',
    'werkzeug<1.1.0',
]
def do_setup():
    """Perform the Airflow package setup."""
    install_providers_from_sources = os.getenv('INSTALL_PROVIDERS_FROM_SOURCES')
    exclude_patterns = \
        [] if install_providers_from_sources and install_providers_from_sources == 'true' \
        else ['airflow.providers', 'airflow.providers.*']
    write_version()
    setup(
        name='apache-airflow',
        description='Programmatically author, schedule and monitor data pipelines',
        long_description=long_description,
        long_description_content_type='text/markdown',
        license='Apache License 2.0',