fix: keep the copyreg module [backport #5750 to 1.13] (#5754)

Backport of #5750 to 1.13

Some frameworks might extend the pickle capabilities by interacting with
the copyreg module. Unloading it as part of the module clean-up process
causes issues, so we add it to the list of modules to keep.

Fixes: #5742 

## Testing strategy

The issue does not occur when the library is installed in edit mode,
which makes it tricky to test with `riot` in our CI. Manual testing
shows that issue that prompted this fix is resolved.

## Risk

Because we don't have a reproducer in our test suite we are left
vulnerable to regressions.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] PR description includes explicit acknowledgement/acceptance of the
performance implications of this PR as reported in the benchmarks PR
comment.

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.

Author: P403n1x87

ddtrace/bootstrap/sitecustomize.py

@@ -124,6 +124,7 @@ def drop(module_name):
     KEEP_MODULES = frozenset(
         [
             "atexit",
+            "copyreg",  # pickling issues for tracebacks with gevent
             "ddtrace",
             "asyncio",
             "concurrent",
@@ -134,7 +135,7 @@ def drop(module_name):
         ]
     )
     if PY2:
-        KEEP_MODULES_PY2 = frozenset(["encodings", "codecs"])
+        KEEP_MODULES_PY2 = frozenset(["encodings", "codecs", "copy_reg"])
     for m in list(_ for _ in sys.modules if _ not in LOADED_MODULES):
         if any(m == _ or m.startswith(_ + ".") for _ in KEEP_MODULES):
             continue

releasenotes/notes/fix-keep-copyreg-29c28bcc854e13bb.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    gevent: Fix a bug that caused traceback objects to fail to pickle when using gevent.