fix(tracer): add importlib._bootstrap to keep modules (#15131)

## Description

applications instrumented by ddtrace-run could fail to start with PEP420
style modules. This fixes that problem.

## Testing

System tests with DataDog/system-tests#5641

## Additional Notes

APPSEC-59557

Author: christophe-papazian

ddtrace/bootstrap/cloning.py

@@ -69,6 +69,7 @@ def drop(module_name: str) -> None:
             "copyreg",  # pickling issues for tracebacks with gevent
             "ddtrace",
             "concurrent",
+            "importlib._bootstrap",  # special import that must not be unloaded
             "typing",
             "_operator",  # pickling issues with typing module
             "re",  # referenced by the typing module

releasenotes/notes/pep420_compatibility-99ca4ca528ad47d6.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    tracer: This fix resolves an issue where an application instrumented by ddtrace could crash at start.
+    Fix compatibility with zope.event==6.0