fix(gevent): unload time but not typing modules for module cloning (#…

…5135)

This PR makes a fix to #4863 which inadvertently removed unloading the
`time` module as well as removing the `typing` module from the list of
standard library modules to not unload for module cloning. We found that
the `time` module is a special case (which is important to unload due to
interactions with gevent patching) as it is implicitly imported by
recent versions of CPython on interpreter startup, so it may be present
in `sys.modules` already.
The `typing` module is also problematic on being reloaded for older
versions of Python < 3.7, so we should not unload/reload this module as
well.

ddtrace/bootstrap/sitecustomize.py

@@ -12,7 +12,14 @@
 import os  # noqa
 
 
+"""
+The following modules cause problems when being unloaded/reloaded in module cloning.
+Notably, unloading the atexit module will remove all registered hooks which we use for cleaning up on tracer shutdown.
+The other listed modules internally maintain some state that does not coexist well if reloaded.
+"""
 MODULES_TO_NOT_CLEANUP = {"atexit", "asyncio", "attr", "concurrent", "ddtrace", "logging"}
+if sys.version_info < (3, 7):
+    MODULES_TO_NOT_CLEANUP |= {"typing"}  # required by older versions of Python
 if sys.version_info <= (2, 7):
     MODULES_TO_NOT_CLEANUP |= {"encodings", "codecs"}
     import imp
@@ -72,6 +79,9 @@ def cleanup_loaded_modules(aggressive=False):
                 break
         else:
             del sys.modules[module_name]
+    # Some versions of CPython import the time module during interpreter startup, which needs to be unloaded.
+    if "time" in sys.modules:
+        del sys.modules["time"]
 
 
 will_run_module_cloning = should_cleanup_loaded_modules()