fix: pin transformers<5.13.0 to avoid AutoTokenizer.register breakage

[rggammon]

Problem

transformers==5.13.0 introduced a breaking change in AutoTokenizer.register() that causes mlx-lm to fail at import time with Python 3.14 on macOS.

The error occurs in mlx_lm/tokenizer_utils.py at:
python AutoTokenizer.register("NewlineTokenizer", fast_tokenizer_class=NewlineTokenizer)

transformers 5.13.0 added a check in auto_factory.py that requires registered tokenizer classes to have module starting with 'transformers.'. Since NewlineTokenizer is defined in mlx-lm, not in transformers, this check fails.

Error:

File "...transformers/models/auto/auto_factory.py", line 680, in register
    if key.__module__.startswith("transformers."):

To reproduce:

pip install mlx-lm transformers==5.13.0
python -c "import mlx_lm"  # fails

Fix

Pin transformers<5.13.0 as a temporary fix while a proper solution (e.g. updating the AutoTokenizer.register call to be compatible with the new API) is investigated.