Make all models frozen, and sessions hashable

tests/conftest.py

@@ -1,13 +1,9 @@
 import pytest
+import fbchat
 
 
 @pytest.fixture(scope="session")
 def session():
-    class FakeSession:
-        # TODO: Add a further mocked session
-        user_id = "31415926536"
-
-        def __repr__(self):
-            return "<FakeSession>"
-
-    return FakeSession()
+    return fbchat.Session(
+        user_id="31415926536", fb_dtsg=None, revision=None, session=None
+    )