Remove conditional typing_extensions import for python < 3.8

This repository doesn't currently, and never has, supported python versions
less than 3.8. The conditional import being removed here is a holdover from
when this code was cut out of dbt-core (which no longer supports python less
than 3.8 either).

dbt_common/events/base_types.py

@@ -10,10 +10,7 @@
 
 from dbt_common.invocation import get_invocation_id
 
-if sys.version_info >= (3, 8):
-    from typing import Protocol
-else:
-    from typing_extensions import Protocol
+from typing import Protocol
 
 
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