Resolve a circular import error in obscure configuarations.

Signed-off-by: Jared Hance <jaredhance@gmail.com>

Author: jhance

src/memray/_metadata.py

@@ -1,7 +1,11 @@
+from __future__ import annotations
+
+import typing
 from dataclasses import dataclass
 from datetime import datetime
 
-from ._memray import FileFormat
+if typing.TYPE_CHECKING:
+    from ._memray import FileFormat
 
 
 @dataclass