Improve error message for eventlet/gevent monkey patching (#3140)

* Improve error message for eventlet/gevent monkey patching

* Add a newsfragment

* Update src/trio/_core/_run.py

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

* Clean up a bit

---------

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

Author: A5rocks

newsfragments/3087.doc.rst

@@ -0,0 +1 @@
+Improve error message when run after gevent's monkey patching.

src/trio/_core/_run.py

@@ -2953,6 +2953,13 @@ async def checkpoint_if_cancelled() -> None:
         _KqueueStatistics as IOStatistics,
     )
 else:  # pragma: no cover
+    _patchers = sorted({"eventlet", "gevent"}.intersection(sys.modules))
+    if _patchers:
+        raise NotImplementedError(
+            "unsupported platform or primitives trio depends on are monkey-patched out by "
+            + ", ".join(_patchers),
+        )
+
     raise NotImplementedError("unsupported platform")
 
 from ._generated_instrumentation import *