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
'paramiko',
    'pipdeptree',
    'pre-commit',
    'pylint==2.5.3',
    'pysftp',
    'pytest',
    'pytest-cov',
    'pytest-instafail',
    'pytest-rerunfailures',
    'pytest-timeouts',
    'pytest-xdist',
    'pywinrm',
    'qds-sdk>=1.9.6',
    'requests_mock',
    'setuptools',
    'testfixtures',
    'wheel',
    'yamllint',
]
if PY3:
    devel += ['mypy==0.770']
else:
    devel += ['unittest2']
devel_minreq = cgroups + devel + doc + kubernetes + mysql + password
devel_hadoop = devel_minreq + hdfs + hive + kerberos + presto + webhdfs
PROVIDERS_REQUIREMENTS: Dict[str, Iterable[str]] = {
    "amazon": amazon,
    "apache.cassandra": cassandra,
    "apache.druid": druid,
    "apache.hdfs": hdfs,