Remove various bits of compatibility code for Python <3.6 (#9879)

I went through and removed a bunch of cruft that was lying around for compatibility with old Python versions. This PR also will now prevent Synapse from starting unless you're running Python 3.6+.

tests/rest/admin/test_user.py

@@ -18,7 +18,7 @@ import json
 import urllib.parse
 from binascii import unhexlify
 from typing import List, Optional
-from unittest.mock import Mock
+from unittest.mock import Mock, patch
 
 import synapse.rest.admin
 from synapse.api.constants import UserTypes
@@ -54,8 +54,6 @@ class UserRegisterTestCase(unittest.HomeserverTestCase):
         self.datastore = Mock(return_value=Mock())
         self.datastore.get_current_state_deltas = Mock(return_value=(0, []))
 
-        self.secrets = Mock()
-
         self.hs = self.setup_test_homeserver()
 
         self.hs.config.registration_shared_secret = "shared"
@@ -84,14 +82,13 @@ class UserRegisterTestCase(unittest.HomeserverTestCase):
         Calling GET on the endpoint will return a randomised nonce, using the
         homeserver's secrets provider.
         """
-        secrets = Mock()
-        secrets.token_hex = Mock(return_value="abcd")
+        with patch("secrets.token_hex") as token_hex:
+            # Patch secrets.token_hex for the duration of this context
+            token_hex.return_value = "abcd"
 
-        self.hs.get_secrets = Mock(return_value=secrets)
+            channel = self.make_request("GET", self.url)
 
-        channel = self.make_request("GET", self.url)
-
-        self.assertEqual(channel.json_body, {"nonce": "abcd"})
+            self.assertEqual(channel.json_body, {"nonce": "abcd"})
 
     def test_expired_nonce(self):
         """

tests/storage/test__base.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import secrets
 
 from tests import unittest
 
@@ -21,7 +22,7 @@ class UpsertManyTests(unittest.HomeserverTestCase):
     def prepare(self, reactor, clock, hs):
         self.storage = hs.get_datastore()
 
-        self.table_name = "table_" + hs.get_secrets().token_hex(6)
+        self.table_name = "table_" + secrets.token_hex(6)
         self.get_success(
             self.storage.db_pool.runInteraction(
                 "create",

tests/unittest.py

@@ -18,6 +18,7 @@ import hashlib
 import hmac
 import inspect
 import logging
+import secrets
 import time
 from typing import Callable, Dict, Iterable, Optional, Tuple, Type, TypeVar, Union
 from unittest.mock import Mock, patch
@@ -626,7 +627,6 @@ class HomeserverTestCase(TestCase):
             str: The new event's ID.
         """
         event_creator = self.hs.get_event_creation_handler()
-        secrets = self.hs.get_secrets()
         requester = create_requester(user)
 
         event, context = self.get_success(