Update to relative import in browsergym_env

[sergiopaniego]

The current approach generates:

OpenEnv/src/envs/browsergym_env/client.py", line 6, in <module>
    from browsergym_env.models import (
ModuleNotFoundError: No module named 'browsergym_env'

when imported the env when installed via pip

cc @Paulescu

[sergiopaniego]

@init27 @pankit-eng @jspisak 😄

[burtenshaw]

@sergiopaniego are you installing the env with uv pip install . ?