Added some type checks before assigning to session['blender_id_oauth_token']

There were some sporadic TypeErrors where the session var was set to a
tuple instead of a string; this is a way to figure out where that happens.

pillar/auth/oauth.py

@@ -154,7 +154,10 @@ class BlenderIdSignIn(OAuthSignIn):
         oauth_session = self.make_oauth_session()
 
         # TODO handle exception for failed oauth or not authorized
-        session['blender_id_oauth_token'] = oauth_session.access_token
+        access_token = oauth_session.access_token
+        assert isinstance(access_token, str), f'oauth token must be str, not {type(access_token)}'
+
+        session['blender_id_oauth_token'] = access_token
         me = oauth_session.get('user').json()
         return OAuthUserResponse(str(me['id']), me['email'])