build: move pygithub to release group to fix free-threaded wheel builds

pygithub pulls in cryptography via pyjwt[crypto]. cryptography 44.0.0
ships only abi3 wheels, which free-threaded interpreters cannot use, so
uv builds it from sdist; its bundled PyO3 0.23.2 caps at Python 3.13 and
fails on 3.14t. pygithub is only used by the manual release changelog
script, so move it out of the dev group into a new release group.
'uv sync --dev' (used by CI test jobs) no longer drags in cryptography.

Author: timsaucer

dev/release/README.md

@@ -75,6 +75,7 @@ We maintain a `CHANGELOG.md` so our users know what has been changed between rel
 The changelog is generated using a Python script:
 
 ```bash
+$ uv sync --group release
 $ GITHUB_TOKEN=<TOKEN> ./dev/release/generate-changelog.py 52.0.0 HEAD 53.0.0 > dev/changelog/53.0.0.md
 ```
 

pyproject.toml

@@ -202,13 +202,18 @@ dev = [
   "numpy>=2.3.2;python_version>='3.14'",
   "pre-commit>=4.3.0",
   "pyarrow>=19.0.0",
-  "pygithub==2.5.0",
   "pytest-asyncio>=0.23.3",
   "pytest>=7.4.4",
   "pyyaml>=6.0.3",
   "ruff>=0.15.1",
   "toml>=0.10.2",
 ]
+# Release tooling only. Kept out of `dev` because pygithub pulls in
+# cryptography, which ships no free-threaded wheel and fails to build
+# from sdist under free-threaded interpreters (PyO3 < 3.14 support).
+release = [
+  "pygithub==2.5.0",
+]
 docs = [
   "ipython>=8.12.3",
   "jinja2>=3.1.5",