Scope greenlet constraint to Python 3.13+

pyproject.toml

@@ -17,7 +17,7 @@ dependencies = [
     "browsergym-miniwob==0.14.3",
     "gymnasium>=0.29.0",
     "playwright>=1.40.0",
-    "greenlet>=3.1.0",  # Required for Python 3.13 compatibility
+    "greenlet>=3.1.0; python_version >= '3.13'",  # Required only for Python 3.13 compatibility
     "Pillow>=10.0.0",
 ]