build(setup): pin sagemaker<3 so import cohere_aws works again

[tsushanth]

Summary

Closes #233.

sagemaker 3.0 removed the sagemaker.s3 module — S3Downloader, S3Uploader, and parse_s3_url are no longer importable from that path. cohere_aws/client.py:12 imports all three at module load:

from sagemaker.s3 import S3Downloader, S3Uploader, parse_s3_url

So any environment that resolves to sagemaker>=3 throws ModuleNotFoundError on the first import cohere_aws, long before the caller has a chance to do anything else:

>>> import cohere_aws
ModuleNotFoundError: No module named 'sagemaker.s3'

setup.py declared the dep as unpinned sagemaker, which is now wrong on any fresh install that picks the latest published version.

Fix

-                 install_requires=['boto3>=1.34.162', 'sagemaker'],
+                 # sagemaker 3.0 removed the `sagemaker.s3` module
+                 # (S3Downloader / S3Uploader / parse_s3_url), which client.py
+                 # imports unconditionally — installing this package alongside
+                 # sagemaker>=3 raises ModuleNotFoundError on first import.
+                 # Pin to <3 until the imports are migrated to sagemaker v3's
+                 # surface. See https://github.com/cohere-ai/cohere-aws/issues/233.
+                 install_requires=['boto3>=1.34.162', 'sagemaker<3'],

Matches what @robertkerr-cohere suggested on the issue thread:

This is caused by using sagemaker version 3, which removed the sagemaker.s3 path. This package should pin sagemaker<3 or upgrade code for v3.

This PR takes the first half of his suggestion — restore importability today. The longer-term migration to sagemaker v3's APIs (where S3Downloader / S3Uploader moved to sagemaker.s3_utils, etc.) is a separate, larger change worth tracking in its own issue.

The inline comment links #233 so a future maintainer trying to lift the constraint sees the migration prerequisite right there.

Test plan

• Manual: a fresh pip install cohere-aws against this branch now resolves sagemaker<3 and import cohere_aws succeeds. On main the same flow either fails at install (if pip happened to pick sagemaker>=3) or at first import (if a newer sagemaker was already in the env).
• boto3>=1.34.162 constraint left untouched — only the sagemaker dep is changed.