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
"""
    Checks if package should be excluded.
    :param package: package name (beginning of it)
    :param exclusion_list: list of excluded packages
    :return: true if package should be excluded
    """
    return any(package.startswith(excluded_package) for excluded_package in exclusion_list)
devel_all = [package for package in devel_all if not is_package_excluded(
    package=package,
    exclusion_list=PACKAGES_EXCLUDED_FOR_ALL)
]
devel_ci = [package for package in devel_all if not is_package_excluded(
    package=package,
    exclusion_list=PACKAGES_EXCLUDED_FOR_CI + PACKAGES_EXCLUDED_FOR_ALL)
]
EXTRAS_REQUIREMENTS.update(
    {
        'all': devel_all,
        'devel_ci': devel_ci,
    }
)
INSTALL_REQUIREMENTS = [
    'alembic>=1.2, <2.0',
    'argcomplete~=1.10',
    'attrs~=20.0',
    'cached_property~=1.5',