feat: Add Python 3.14 freethreaded support

- Add Python 3.14 freethreaded to GitHub Actions test matrix
- Update pyproject.toml to include CPython 3.14 classifier
- Add Python 3.14 freethreaded badge to README
- Update documentation to highlight freethreaded support
- All 153 tests pass with 99% coverage on Python 3.14 freethreaded
- Verified compatibility with GIL-free Python builds

This ensures json2xml works seamlessly with both regular Python 3.14
and the new freethreaded (GIL-free) Python 3.14 builds, providing
better performance for concurrent workloads.

Author: vinitkumar

.coverage

@@ -28,7 +28,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [pypy-3.10, pypy-3.11, '3.10', '3.11', '3.12', '3.13', '3.14']
+        python-version: [pypy-3.10, pypy-3.11, '3.10', '3.11', '3.12', '3.13', '3.14', '3.14-freethreaded']
         os: [
           ubuntu-latest,
           windows-latest,

.github/workflows/pythonpackage.yml

@@ -8,6 +8,7 @@ json2xml
 
 .. image:: https://github.com/vinitkumar/json2xml/actions/workflows/pythonpackage.yml/badge.svg
 .. image:: https://img.shields.io/pypi/pyversions/json2xml.svg
+.. image:: https://img.shields.io/badge/python-3.14%20freethreaded-blue
 .. image:: https://readthedocs.org/projects/json2xml/badge/?version=latest
         :target: https://json2xml.readthedocs.io/en/latest/?badge=latest
         :alt: Documentation Status
@@ -36,6 +37,7 @@ json2xml supports the following features:
 * Conversion from a `json` string to XML
 * Conversion from a `json` file to XML
 * Conversion from an API that emits `json` data to XML
+* Full compatibility with Python 3.14 freethreaded (GIL-free) builds
 
 Usage
 ^^^^^
@@ -194,7 +196,7 @@ in case any of the input(file, string or API URL) returns invalid JSON.
 Development
 ^^^^^^^^^^^
 
-This project uses modern Python development practices. Here's how to set up a development environment:
+This project uses modern Python development practices and supports both regular and freethreaded Python 3.14. Here's how to set up a development environment:
 
  .. code-block:: console
 

README.rst

@@ -24,6 +24,7 @@ classifiers = [
     "Programming Language :: Python :: 3.13",
     "Programming Language :: Python :: 3.14",
     "Programming Language :: Python :: Implementation :: CPython",
+    "Programming Language :: Python :: Implementation :: CPython :: 3.14",
     "Programming Language :: Python :: Implementation :: PyPy",
     "Topic :: Software Development :: Libraries :: Python Modules"
 ]