Keep runtime fallback for 3.7/3.8 compat

Because `Mapping` is not subscriptable until Python 3.9, the previous change introduced runtime errors for 3.7/3.8. Applies suggested workaround from @WilliamBergamin to continue using Dict at runtime until <3.9 is eventually dropped.

Author: BHSPitMonkey

slack_sdk/signature/__init__.py

@@ -2,9 +2,14 @@
 
 import hashlib
 import hmac
-from collections.abc import Mapping
 from time import time
-from typing import Optional, Union
+from typing import Dict, Optional, Union, TYPE_CHECKING
+
+# Fallback to Dict for Python 3.7/3.8 compatibility (safe to remove once these versions are no longer supported)
+if TYPE_CHECKING:
+    from collections.abc import Mapping
+else:
+    Mapping = Dict
 
 
 class Clock: