Replace serde with stdlib dataclasses, bump to 2.0.0

The serde library is unmaintained. Replace it with Python stdlib
dataclasses and a custom Model base class that provides the same
from_dict()/to_dict() API with identical behavior:
- Optional fields omitted from to_dict() when None
- Extra/unknown fields silently ignored
- ValidationError on missing required fields

Also adds Python 3.14 to CI matrix and removes serde-specific
mypy overrides.

Author: dannywillems

.github/workflows/action.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.11", "3.12", "3.13"]
+        python-version: ["3.11", "3.12", "3.13", "3.14"]
         os: ["ubuntu-latest", "macos-latest", "windows-latest"]
     runs-on: ${{ matrix.os }}
     steps:

l9format/__init__.py

@@ -33,6 +33,7 @@
     ServiceCredentials,
     Software,
     SoftwareModule,
+    ValidationError,
 )
 
 __all__ = [
@@ -66,4 +67,5 @@
     "ServiceCredentials",
     "Software",
     "SoftwareModule",
+    "ValidationError",
 ]