fix: keep the sre_constants module [backport #6255 to 1.14] (#6323)

Backport of #6255 to 1.14

The re module imports from the sre_constants module at runtime. This
might cause issues when modules are unloaded in the sitecustomize.

Fixes #6197.

## 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/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## 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.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: P403n1x87

ddtrace/bootstrap/sitecustomize.py

@@ -109,6 +109,7 @@ def drop(module_name):
             "concurrent",
             "typing",
             "re",  # referenced by the typing module
+            "sre_constants",  # imported by re at runtime
             "logging",
             "attr",
             "google.protobuf",  # the upb backend in >= 4.21 does not like being unloaded

releasenotes/notes/fix-re-unload-8770a3d497234243.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a bug that caused applications using gevent and cassandra to fail to
+    start with the ddtrace-run command.

tests/commands/test_runner.py

@@ -468,3 +468,16 @@ def test_no_args():
     p.wait()
     assert p.returncode == 1
     assert six.b("usage:") in p.stdout.read()
+
+
+@pytest.mark.subprocess(ddtrace_run=True, env=dict(DD_UNLOAD_MODULES_FROM_SITECUSTOMIZE="1"))
+def test_ddtrace_re_module():
+    import re
+
+    re.Scanner(
+        (
+            ("frozen", None),
+            (r"[a-zA-Z0-9_]+", lambda s, t: t),
+            (r"[\s,<>]", None),
+        )
+    )