fix: Use .mappings() in cursor, to get rows as mappings (#309)

~This was fixed upstream in meltano/sdk#1487,
but this tap overrides `get_records`.~

I think meltano/sdk#2094 is the only difference
between the two implementations?

UPDATE: Using the private `._mapping` attribute makes me nervous so I
opened meltano/sdk#2095 and updated this PR.

tap_postgres/client.py

@@ -282,7 +282,7 @@ def get_records(self, context: Optional[dict]) -> Iterable[Dict[str, Any]]:
                 query = query.filter(replication_key_col >= start_val)
 
         with self.connector._connect() as con:
-            for row in con.execute(query):
+            for row in con.execute(query).mappings():
                 yield dict(row)