Add missing type declarations for top-level files in src/openfermion

[mhucka]

This adds more type hints to the Python files at the top level of src/openfermion.

This partly addresses #1282.

[mhucka]

/gemini review

[gemini-code-assist]

Code Review

This pull request enhances type safety by adding type annotations to _compat.py, _compat_test.py, and conftest.py. Specifically, it introduces return type hints, argument types, and necessary type ignore comments for dynamic attributes, while also cleaning up unused imports. I have no feedback to provide.

[gemini-code-assist]

Code Review

This pull request adds type hints to _compat.py, _compat_test.py, and conftest.py to enhance static type checking. The changes include specifying return types for functions and methods, adding parameter types, and including type ignore comments for dynamic attribute assignments in tests. Additionally, an unused import was removed from the test suite. I have no feedback to provide.