fix: add future annotations for Python 3.9 compatibility

Author: blackwell-systems

src/gcf/decode.py

@@ -1,5 +1,7 @@
 """GCF decoder: parses GCF text back into a Payload."""
 
+from __future__ import annotations
+
 from .constants import KIND_EXPAND
 from .types import Edge, Payload, Symbol
 

src/gcf/encode.py

@@ -1,5 +1,7 @@
 """GCF encoder: serializes Payload into GCF text format."""
 
+from __future__ import annotations
+
 from .constants import KIND_ABBREV
 from .types import Payload, Symbol
 

src/gcf/generic.py

@@ -1,5 +1,7 @@
 """GCF generic encoder: serializes arbitrary Python values into GCF tabular format."""
 
+from __future__ import annotations
+
 from typing import Any
 
 

src/gcf/session.py

@@ -1,5 +1,7 @@
 """Session-based deduplication for GCF encoding."""
 
+from __future__ import annotations
+
 import threading
 
 from .constants import KIND_ABBREV

src/gcf/types.py

@@ -1,5 +1,7 @@
 """Data types for GCF payloads."""
 
+from __future__ import annotations
+
 from dataclasses import dataclass, field