Use format for string formatting

This package is still compatible and used with Python 2.7 processing environments.

It's also used in production which is running Python 3.5.

This makes the use of the new "fast" string notation not yet possible.

beat/backend/python/database.py

@@ -314,7 +314,11 @@ class Database(object):
         elif schema_version == 2:
             self._load_v2(dataformat_cache)
         else:
-            raise RuntimeError(f"Invalid schema version {schema_version}")
+            raise RuntimeError(
+                "Invalid schema version {schema_version}".format(
+                    schema_version=schema_version
+                )
+            )
 
     @property
     def name(self):