Mark gym_py as not to be upgraded. (RobotLocomotion#18534)

tools/workspace/gym_py/README.md

@@ -2,3 +2,23 @@ gym
 ===
 
 This imports the OpenAI Gym RL tool into our build.
+
+
+Note on Upgrading
+=================
+
+`stable_baselines3` lags `gym` by more than `gym`'s deprecation window and
+does not document its compatiblity limit correctly (claims to support `gym`
+<0.24 but in fact uses classes removed in 0.22, for example).
+
+Because of this, it is unlikely that upgrading this package beyond 0.21 will
+succeed, or is even desirable.
+
+ * Check https://github.com/DLR-RM/stable-baselines3/pull/780 for status.
+ * `stable_baselines3` allegedly will fix all of this version shear when gym
+   hits 1.0.
+ * `gym` will allegedly remove all the deprecated APIs that SB3 uses in 0.26.
+
+A full upgrade of `gym` to 1.0 and correspondingly of `stable_baselines3` to
+HEAD will be necessary in the future, but will require substantial changes to
+the `stable_baselines3_internal` stub and to any corresponding unit tests.

tools/workspace/new_release.py

@@ -55,6 +55,7 @@
 _IGNORED_REPOSITORIES = [
     # We don't know how to check non-default branches yet.
     "clang_cindex_python3_internal",
+    "gym_py",  # Pinned at 0.21; see tools/workspace/gym_py/README.md.
     "pybind11",
 ]