Fix re.Pattern mypy error on 3.6

changelog.d/9703.misc

@@ -0,0 +1 @@
+Fix undetected mypy error when using Python 3.6.

synapse/logging/opentracing.py

@@ -169,7 +169,7 @@ def set_fates(clotho, lachesis, atropos, father="Zues", mother="Themis"):
 import logging
 import re
 from functools import wraps
-from typing import TYPE_CHECKING, Dict, Optional, Type
+from typing import TYPE_CHECKING, Dict, Optional, Pattern, Type
 
 import attr
 
@@ -262,7 +262,7 @@ def report_span(self, span):
 # Block everything by default
 # A regex which matches the server_names to expose traces for.
 # None means 'block everything'.
-_homeserver_whitelist = None  # type: Optional[re.Pattern[str]]
+_homeserver_whitelist = None  # type: Optional[Pattern[str]]
 
 # Util methods