title: Webapp2 Python set_cookie does not support samesite cookie?
body: <p>In <a href="https://webapp2.readthedocs.io/en/latest/guide/response.html#setting-cookies" rel="noreferrer">webapp2 documentation</a> there is no mention of setting the <code>SameSite</code> attribute for a cookie, it seems to be built on the response handler from WebOB, I checked webOB doc page it clearly shows the <a href="https://docs.pylonsproject.org/projects/webob/en/stable/api/cookies.html" rel="noreferrer">'SameSite' flag as an accepted cookie parameter</a></p>
  
  <p>I tried to set it nonetheless in set cookie:</p>
  
  <pre><code>self.response.set_cookie(name, secure_cookie, path='/', secure=True,
  httponly=True, samesite='lax', expires=expireDate)
  </code></pre>
  
  <p>But I received the below error:</p>
  
  <p><code>TypeError: set_cookie() got an unexpected keyword argument 'samesite'</code></p>
  
  <p>I know one can use <code>self.response.headers.add_header('Set-Cookie', ...</code> but I was hoping I could use <code>self.response.set_cookie</code> following the webapp2 documentation </p>