Avoid using mutable class attributes

A tuple works fine here.

vdirsyncer/storage/base.py

@@ -75,7 +75,7 @@ class Storage(metaclass=StorageMeta):
     read_only = False
 
     # The attribute values to show in the representation of the storage.
-    _repr_attributes: list[str] = []
+    _repr_attributes: tuple[str, ...] = ()
 
     def __init__(
         self,

vdirsyncer/storage/dav.py

@@ -451,7 +451,7 @@ class DAVStorage(Storage):
 
     connector: aiohttp.TCPConnector
 
-    _repr_attributes = ["username", "url"]
+    _repr_attributes = ("username", "url")
 
     _property_table = {
         "displayname": ("displayname", "DAV:"),

vdirsyncer/storage/filesystem.py

@@ -22,7 +22,7 @@ logger = logging.getLogger(__name__)
 
 class FilesystemStorage(Storage):
     storage_name = "filesystem"
-    _repr_attributes = ["path"]
+    _repr_attributes = ("path",)
 
     def __init__(
         self,

vdirsyncer/storage/http.py

@@ -23,7 +23,7 @@ logger = logging.getLogger(__name__)
 class HttpStorage(Storage):
     storage_name = "http"
     read_only = True
-    _repr_attributes = ["username", "url"]
+    _repr_attributes = ("username", "url")
     _items = None
 
     # Required for tests.

vdirsyncer/storage/singlefile.py

@@ -40,7 +40,7 @@ def _writing_op(f):
 
 class SingleFileStorage(Storage):
     storage_name = "singlefile"
-    _repr_attributes = ["path"]
+    _repr_attributes = ("path",)
 
     _write_mode = "wb"
     _append_mode = "ab"