Migrate packaging metadata to pyproject.toml (PEP 621)

Replace setup.py and Pipenv files with a complete PEP 621 pyproject.toml:
- Full [project] table with metadata, classifiers, optional-deps
- Dynamic version via setuptools AST parsing of serpapi/__version__.py
- pytest configuration in [tool.pytest.ini_options]
- Remove setup.py, Pipfile, and Pipfile.lock

Author: ilyazub