feat(fsspec): FsspecStore.modified() (#517)

* FsspecStore.modified()

* import: datetime --> TYPE_CHECKING

* ruff formatting

Author: keen85

obstore/python/obstore/fsspec.py

@@ -50,6 +50,7 @@
 if TYPE_CHECKING:
     import sys
     from collections.abc import Coroutine, Iterable
+    from datetime import datetime
     from typing import Any
 
     from obstore import Attributes, Bytes, ReadableFile, WritableFile
@@ -591,6 +592,13 @@ def _open(
 
         return BufferedFile(self, path, mode, **kwargs)
 
+    def modified(self, path: str) -> datetime:
+        """Return the modified timestamp of a file as a `datetime.datetime`."""
+        bucket, path_no_bucket = self._split_path(path)
+        store = self._construct_store(bucket)
+        head = obs.head(store, path_no_bucket)
+        return head["last_modified"]
+
 
 class BufferedFile(fsspec.spec.AbstractBufferedFile):
     """A buffered readable or writable file.