MAINT: remove deprecated notebooks utility and requests dependency

[AdityaBil]

Closes #954

Description
This PR addresses the v1.0 maintenance milestone by removing the deprecated notebooks utility and dropping requests from the package dependencies.

Changes Made:

• Deleted quantecon/util/notebooks.py and its associated test files (test_notebooks.py, test_file.md).
• Cleaned up fetch_nb_dependencies imports and __all__ declarations in quantecon/__init__.py and quantecon/util/__init__.py.
• Removed quantecon.util.notebooks from SKIP_MODULES in test_docstring_examples.py.
• Removed the requests dependency from pyproject.toml, environment.yml, environment_np2.yml, and docs/rtd-requirements.txt.

Checklist:

• Verified no active code references to requests remain.
• All tests pass locally (pytest quantecon/tests/test_docstring_examples.py).