fix: enforce thread safety in sysmon.py

Testing on nogil failed with:

```
2024-06-13T07:28:52.5961120Z     @panopticon()
2024-06-13T07:28:52.5962049Z     def stop(self) -> None:
2024-06-13T07:28:52.5963007Z         """Stop this Tracer."""
2024-06-13T07:28:52.5963993Z         if not self.sysmon_on:
2024-06-13T07:28:52.5964998Z             # In forking situations, we might try to stop when we are not
2024-06-13T07:28:52.5966053Z             # started.  Do nothing in that case.
2024-06-13T07:28:52.5966980Z             return
2024-06-13T07:28:52.5967980Z         assert sys_monitoring is not None
2024-06-13T07:28:52.5968885Z         sys_monitoring.set_events(self.myid, 0)
2024-06-13T07:28:52.5969793Z         self.sysmon_on = False
2024-06-13T07:28:52.5970648Z >       for code in self.local_event_codes.values():
2024-06-13T07:28:52.5971663Z E       RuntimeError: dictionary changed size during iteration
2024-06-13T07:28:52.5972502Z
2024-06-13T07:28:52.5972851Z coverage/sysmon.py:252: RuntimeError
```

Author: nedbat

coverage/sysmon.py

@@ -200,6 +200,7 @@ def __init__(self, tool_id: int) -> None:
         # Map id(code_object) -> code_object
         self.local_event_codes: dict[int, CodeType] = {}
         self.sysmon_on = False
+        self.lock = threading.Lock()
 
         self.stats = {
             "starts": 0,
@@ -248,10 +249,11 @@ def stop(self) -> None:
             return
         assert sys_monitoring is not None
         sys_monitoring.set_events(self.myid, 0)
-        self.sysmon_on = False
-        for code in self.local_event_codes.values():
-            sys_monitoring.set_local_events(self.myid, code, 0)
-        self.local_event_codes = {}
+        with self.lock:
+            self.sysmon_on = False
+            for code in self.local_event_codes.values():
+                sys_monitoring.set_local_events(self.myid, code, 0)
+            self.local_event_codes = {}
         sys_monitoring.free_tool_id(self.myid)
 
     @panopticon()
@@ -332,20 +334,21 @@ def sysmon_py_start(self, code: CodeType, instruction_offset: int) -> MonitorRet
 
             if tracing_code:
                 events = sys.monitoring.events
-                if self.sysmon_on:
-                    assert sys_monitoring is not None
-                    sys_monitoring.set_local_events(
-                        self.myid,
-                        code,
-                        events.PY_RETURN
-                        #
-                        | events.PY_RESUME
-                        # | events.PY_YIELD
-                        | events.LINE,
-                        # | events.BRANCH
-                        # | events.JUMP
-                    )
-                    self.local_event_codes[id(code)] = code
+                with self.lock:
+                    if self.sysmon_on:
+                        assert sys_monitoring is not None
+                        sys_monitoring.set_local_events(
+                            self.myid,
+                            code,
+                            events.PY_RETURN
+                            #
+                            | events.PY_RESUME
+                            # | events.PY_YIELD
+                            | events.LINE,
+                            # | events.BRANCH
+                            # | events.JUMP
+                        )
+                        self.local_event_codes[id(code)] = code
 
         if tracing_code and self.trace_arcs:
             frame = self.callers_frame()