fix _db_command: TypeError: a bytes-like object is required, not 'list'

Bytes come in as an array of integers, which we could probably solve on
the PyO3 end instead, but this will do for now - the same fix is already
used for non-DB case.

https://forums.ankiweb.net/t/anki-2-1-50-beta/15608/15

pylib/anki/_backend/__init__.py

@@ -97,7 +97,7 @@ class RustBackend(RustBackendGenerated):
         try:
             return from_json_bytes(self._backend.db_command(bytes_input))
         except Exception as error:
-            err_bytes = error.args[0]
+            err_bytes = bytes(error.args[0])
         err = backend_pb2.BackendError()
         err.ParseFromString(err_bytes)
         raise backend_exception_to_pylib(err)