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
"""
    Compile and build the frontend assets using yarn and webpack.
    Registered as cmdclass in setup() so it can be called with ``python setup.py compile_assets``.
    """
    description = "Compile and build the frontend assets"
    user_options = []  
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):
        """Set final values for options."""
    def run(self):  
        """Run a command to compile and build assets."""
        subprocess.check_call('./airflow/www/compile_assets.sh')
class ListExtras(Command):
    """
    List all available extras
    Registered as cmdclass in setup() so it can be called with ``python setup.py list_extras``.
    """
    description = "List available extras"
    user_options = []  
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):