`apitools.base.protorpclite.messages.EnumDefinitionError: May only use integers in Enum definitions. Found: __static_attributes__ = ()` error

All commands on version 5.31 installed via pip are failing with a `apitools.base.protorpclite.messages.EnumDefinitionError: May only use integers in Enum definitions. Found: __static_attributes__ = ()` error.

Take the following simplified Dockerfile:

```dockerfile
FROM public.ecr.aws/docker/library/python:3-bullseye

RUN pip install gsutil
RUN pip freeze | grep gsutil && \
gsutil help
```

The `gsutil help` fails during the build:

```
[+] Building 1.0s (6/6) FINISHED docker:default
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 164B 0.0s
=> [internal] load metadata for public.ecr.aws/docker/library/python:3-bullseye 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [1/3] FROM public.ecr.aws/docker/library/python:3-bullseye 0.0s
=> CACHED [2/3] RUN pip install gsutil 0.0s
=> ERROR [3/3] RUN pip freeze | grep gsutil && gsutil help 0.9s
------
> [3/3] RUN pip freeze | grep gsutil && gsutil help:
0.571 gsutil==5.31
0.874 Traceback (most recent call last):
0.875 File "/usr/local/bin/gsutil", line 5, in
0.875 from gslib.__main__ import main
0.875 File "/usr/local/lib/python3.13/site-packages/gslib/__main__.py", line 89, in
0.875 from gslib import wildcard_iterator
0.875 File "/usr/local/lib/python3.13/site-packages/gslib/wildcard_iterator.py", line 43, in
0.875 from gslib.third_party.storage_apitools import storage_v1_messages as apitools_messages
0.875 File "/usr/local/lib/python3.13/site-packages/gslib/third_party/storage_apitools/storage_v1_messages.py", line 20, in
0.875 from apitools.base.protorpclite import message_types as _message_types
0.875 File "/usr/local/lib/python3.13/site-packages/apitools/base/protorpclite/message_types.py", line 25, in
0.875 from apitools.base.protorpclite import messages
0.875 File "/usr/local/lib/python3.13/site-packages/apitools/base/protorpclite/messages.py", line 549, in
0.875 class Variant(Enum):
0.875 ...<34 lines>...
0.875 SINT64 = 18
0.875 File "/usr/local/lib/python3.13/site-packages/apitools/base/protorpclite/messages.py", line 319, in __init__
0.875 raise EnumDefinitionError(
0.875 ...<2 lines>...
0.875 (attribute, value))
0.875 apitools.base.protorpclite.messages.EnumDefinitionError: May only use integers in Enum definitions. Found: __static_attributes__ = ()
------
Dockerfile:4
--------------------
3 | RUN pip install gsutil
4 | >>> RUN pip freeze | grep gsutil && \
5 | >>> gsutil help
6 |
--------------------
ERROR: failed to solve: process "/bin/sh -c pip freeze | grep gsutil && gsutil help" did not complete successfully: exit code: 1
```

Reproduce the failure with the supplied Python 3.13 Dockerfile and `gsutil help`, then inspect `apitools/base/protorpclite/messages.py` and the import path shown in the traceback. Verify the compatibility issue around Enum definitions, and consider the work done when the command runs without `EnumDefinitionError` in the Docker reproduction.

Written by the indexing model from the issue text.