fix: Python 3.8 compatibility - replace PEP 585 type syntax with typing module imports (#24)

* fix: replace Python 3.9+ type syntax with Python 3.8-compatible typing imports

- Replace dict[K, V] with Dict[K, V] from typing module
- Replace list[T] with List[T] from typing module
- Fixes Python 3.8 compatibility broken by actions/setup-python v6 upgrade
- Affected files: constants.py, utils.py, writer.py
- All tests pass on Python 3.8, 3.9, and 3.12
- Resolves TypeError: 'type' object is not subscriptable on Python 3.8

The codebase used Python 3.9+ PEP 585 syntax (dict[...], list[...]) which
is incompatible with Python 3.8. This was exposed when GitHub Actions
upgraded actions/setup-python from v5 to v6, which installs a stricter
Python 3.8 build that properly enforces type annotation restrictions.

This fix ensures the project delivers on its Python 3.8+ support claim
in pyproject.toml (requires-python = '>=3.8').

* chore: remove FIX_PLAN.md - no longer needed

Author: Justar96

src/toon_format/constants.py

@@ -6,7 +6,7 @@
 the TOON implementation. Centralizes magic values for maintainability.
 """
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Dict
 
 if TYPE_CHECKING:
     from .types import Delimiter
@@ -45,7 +45,7 @@
 # endregion
 
 # region Delimiters
-DELIMITERS: dict[str, "Delimiter"] = {
+DELIMITERS: Dict[str, "Delimiter"] = {
     "comma": COMMA,
     "tab": TAB,
     "pipe": PIPE,

src/toon_format/utils.py

@@ -25,7 +25,7 @@
 
 import functools
 import json
-from typing import Any
+from typing import Any, Dict
 
 # Import encode from parent package (defined in __init__.py before this module is imported)
 # __init__.py defines encode() before importing utils, so this is safe
@@ -91,7 +91,7 @@ def count_tokens(text: str, encoding: str = "o200k_base") -> int:
     return len(enc.encode(text))
 
 
-def estimate_savings(data: Any, encoding: str = "o200k_base") -> dict[str, Any]:
+def estimate_savings(data: Any, encoding: str = "o200k_base") -> Dict[str, Any]:
     """Compare token counts between JSON and TOON formats.
 
     Args:

src/toon_format/writer.py

@@ -6,7 +6,7 @@
 indent string caching for performance.
 """
 
-from typing import List
+from typing import Dict, List
 
 from .types import Depth
 
@@ -24,7 +24,7 @@ def __init__(self, indent_size: int) -> None:
         # Ensure nested structures remain distinguishable even for indent=0
         normalized_indent = indent_size if indent_size > 0 else 1
         self._indentation_string = " " * normalized_indent
-        self._indent_cache: dict[int, str] = {0: ""}
+        self._indent_cache: Dict[int, str] = {0: ""}
         self._indent_size = indent_size
 
     def push(self, depth: Depth, content: str) -> None: