chore: update download_cb type annotation (#38)

src/wags_tails/custom.py

@@ -12,10 +12,22 @@
 
 from collections.abc import Callable
 from pathlib import Path
+from typing import Protocol
 
 from .base_source import DataSource
 
 
+class _DownloadCallbackType(Protocol):
+    """Define type for CustomData ``download_cb`` arg"""
+
+    def __call__(self, version: str, outfile: Path) -> None:
+        """Implicit description of ``download_cb`` arg. Shouldn't actually be used.
+
+        :param version: version to acquire
+        :param outfile: location and filename for final data file
+        """
+
+
 class CustomData(DataSource):
     """Data acquisition class using custom, user-provided acquisition methods."""
 
@@ -24,7 +36,7 @@ def __init__(
         src_name: str,
         filetype: str,
         latest_version_cb: Callable[[], str],
-        download_cb: Callable[[str, Path], None],
+        download_cb: _DownloadCallbackType,
         data_dir: Path | None = None,
         file_name: str | None = None,
         versioned: bool = True,