mypy: Use tuples for muted_topics.

We now have our muted topics use tuples internally,
which allows us to tighten up the annotation
for get_topic_mutes, as well as our schema
checking.

We want to deprecate sub_validator=None
for check_list, so we also introduce
check_tuple here.  Now we also want to deprecate
check_tuple, but it's at least isolated now.

We will use this for data structures that are tuples,
but which are sent as lists over the wire.  Fortunately,
we don't have too many of those.

The plan is to convert tuples to dictionaries,
but backward compatibility may be tricky in some
places.

zerver/lib/validator.py

@@ -163,6 +163,24 @@ def check_list(sub_validator: Optional[Validator[ResultT]]=None, length: Optiona
         return cast(List[ResultT], val)
     return f
 
+def check_tuple(sub_validators: List[Validator[ResultT]]) -> Validator[Tuple[Any, ...]]:
+    def f(var_name: str, val: object) -> Tuple[Any, ...]:
+        if not isinstance(val, tuple):
+            raise ValidationError(_('{var_name} is not a tuple').format(var_name=var_name))
+
+        desired_len = len(sub_validators)
+        if desired_len != len(val):
+            raise ValidationError(_('{var_name} should have exactly {desired_len} items').format(
+                var_name=var_name, desired_len=desired_len,
+            ))
+
+        for i, sub_validator in enumerate(sub_validators):
+            vname = f'{var_name}[{i}]'
+            sub_validator(vname, val[i])
+
+        return val
+    return f
+
 # https://zulip.readthedocs.io/en/latest/testing/mypy.html#using-overload-to-accurately-describe-variations
 @overload
 def check_dict(required_keys: Iterable[Tuple[str, Validator[object]]]=[],