Add environment variable support for Doccano credentials

[ledurnan]

Updated doccano_client tests and project dependencies to utilize environment variables via python-dotenv. This enhancement allows secure and flexible configuration for different environments.

Key Changes

1. Integrated python-dotenv for environment variable management.
2. Replaced hard-coded credentials in test_full_integration_tests.py with environment variables.
3. Added python-dotenv to pyproject.toml.

Testing

Ensure tests pass with .env configurations.