test_threading.SubinterpThreadingTests.test_threads_join crashes on Ubuntu, so disable it.

It doesn't seem to crash locally. Not sure what's going on, but I doubt that's an important use-case (mixing gevent, threads and subinterpreters).

src/gevent/monkey.py

@@ -34,6 +34,11 @@ thread** and **should be done while the program is single-threaded**.
     Patching too late can lead to unreliable behaviour (for example, some
     modules may still use blocking sockets) or even errors.
 
+.. tip::
+
+    Be sure to read the documentation for each patch function to check for
+    known incompatibilities.
+
 Querying
 ========
 
@@ -713,6 +718,10 @@ def patch_thread(threading=True, _threading_local=True, Event=True, logging=True
         :class:`concurrent.futures.ProcessPoolExecutor` (which uses a
         ``Queue``) will hang the process.
 
+        Monkey-patching with this function and using
+        sub-interpreters (and advanced C-level API) and threads may be
+        unstable on certain platforms.
+
     .. versionchanged:: 1.1b1
         Add *logging* and *existing_locks* params.
     .. versionchanged:: 1.3a2

src/gevent/testing/patched_tests_setup.py

@@ -1386,6 +1386,15 @@ if PY310:
         'test_threading.InterruptMainTests.test_can_interrupt_tight_loops',
     ]
 
+    if TRAVIS:
+        disabled_tests += [
+            # The mixing of subinterpreters (with threads) and gevent apparently
+            # leads to a segfault on Ubuntu/GitHubActions/3.10rc1. Not clear why.
+            # But that's not a great use case for gevent.
+            'test_threading.SubinterpThreadingTests.test_threads_join',
+            'test_threading.SubinterpThreadingTests.test_threads_join_2',
+        ]
+
 if TRAVIS:
     disabled_tests += [
         # These tests frequently break when we try to use newer Travis CI images,