Fix docker exec typing

Resolve a latent typing issue introduced in caba607 where the
integration test helper assumed docker exec always returned bytes.
Handle both bytes and streamed output explicitly without changing test
behavior.

Author: lkollar

tests/integration/test_manylinux.py

@@ -358,11 +358,15 @@ def docker_exec(
         env = env.copy() if env is not None else {}
         env["COVERAGE_PROCESS_START"] = "/auditwheel_src/pyproject.toml"
     ec, output = container.exec_run(cmd, workdir=cwd, environment=env)
-    output = output.decode("utf-8")
+    assert isinstance(ec, int)
+    if isinstance(output, bytes):
+        output_text = output.decode("utf-8")
+    else:
+        output_text = b"".join(output).decode("utf-8")
     if ec != expected_retcode:
-        logger.info("docker exec error %s: %s", container.id[:12], output)
-        raise CalledProcessError(ec, cmd, output=output)
-    return output
+        logger.info("docker exec error %s: %s", container.id[:12], output_text)
+        raise CalledProcessError(ec, cmd, output=output_text)
+    return output_text
 
 
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