fix: @lru_cache python 3.7 compatibility

In Python 3.7, which is used in the Tutor CI, the lru_cache needs an argument:
https://docs.python.org/3.7/library/functools.html#functools.lru_cache
On GitHub, CI did not catch this error because it is running Python 3.8.

tutor/utils.py

@@ -163,7 +163,7 @@ def docker(*command: str) -> int:
     return execute("docker", *command)
 
 
-@lru_cache
+@lru_cache(maxsize=None)
 def _docker_compose_command() -> Tuple[str, ...]:
     """
     A helper function to determine which program to call when running docker compose