perf: memoise DataSerializer.read_file results (#543)

* perf: memoise DataSerializer.read_file results

Author: huonw

src/syrupy/extensions/amber/serializer.py

@@ -1,3 +1,4 @@
+import functools
 import os
 from types import GeneratorType
 from typing import (
@@ -79,6 +80,7 @@ def write_file(cls, snapshot_fossil: "SnapshotFossil") -> None:
                     f.write(f"\n{cls._marker_divider}\n")
 
     @classmethod
+    @functools.lru_cache()
     def read_file(cls, filepath: str) -> "SnapshotFossil":
         """
         Read the raw snapshot data (str) from the snapshot file into a dict