chore(tox): Set PYTHONGIL from environment if found

Author: effigies

tox.ini

@@ -60,8 +60,6 @@ pass_env =
   USERNAME
   # Environment variables we check for
   NIPY_EXTRA_TESTS
-  # Python variables
-  PYTHON_GIL
   # Pass user color preferences through
   PY_COLORS
   FORCE_COLOR
@@ -71,7 +69,7 @@ pass_env =
 set_env =
   pre,dev: PIP_EXTRA_INDEX_URL=https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
   pre,dev: UV_EXTRA_INDEX_URL=https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
-  py313t: PYTHONGIL=0
+  py313t: PYTHONGIL={env:PYTHONGIL:0}
 extras =
   test