dspace_statistics_api: Use f-strings instead of format()

We had previously been avoiding the f-strings because we needed to
run on Python 3.5 and they were only available in Python 3.6+, but
now the black formatter requires Python 3.6 and all our systems are
running Python 3.6+ anyways.

dspace_statistics_api/database.py

@@ -15,9 +15,7 @@ class DatabaseManager:
     """Manage database connection."""
 
     def __init__(self):
-        self._connection_uri = "dbname={} user={} password={} host={} port={}".format(
-            DATABASE_NAME, DATABASE_USER, DATABASE_PASS, DATABASE_HOST, DATABASE_PORT
-        )
+        self._connection_uri = f"dbname={DATABASE_NAME} user={DATABASE_USER} password={DATABASE_PASS} host={DATABASE_HOST} port={DATABASE_PORT}"
 
     def __enter__(self):
         try: