Restored the frame hiding hack

Turns out it this is still needed on PyPy.

trio/_core/_run.py

@@ -63,6 +63,29 @@ def _public(fn):
 _r = random.Random()
 
 
+# On CPython, Context.run() is implemented in C and doesn't show up in
+# tracebacks. On PyPy, it is implemented in Python and adds 1 frame to tracebacks.
+def _count_context_run_tb_frames():
+    def function_with_unique_name_xyzzy():
+        1 / 0
+
+    ctx = copy_context()
+    try:
+        ctx.run(function_with_unique_name_xyzzy)
+    except ZeroDivisionError as exc:
+        tb = exc.__traceback__
+        # Skip the frame where we caught it
+        tb = tb.tb_next
+        count = 0
+        while tb.tb_frame.f_code.co_name != "function_with_unique_name_xyzzy":
+            tb = tb.tb_next
+            count += 1
+        return count
+
+
+CONTEXT_RUN_TB_FRAMES = _count_context_run_tb_frames()
+
+
 @attr.s(frozen=True, slots=True)
 class SystemClock:
     # Add a large random offset to our clock to ensure that if people