Remove left-over Python 2-isms

[mhucka]

This removes Python 2 compatibility shims and constructs that were not addressed in a previous PR to remove Python 2 shims.

• Removed from builtins import map, zip in src/openfermion/utils/operator_utils.py. These were legacy imports from the future library to provide Python 3 behavior in Python 2.

• Removed explicit (object) inheritance from all class definitions.

• Removed the redundant u prefix from string literals, because in Python 3, all strings are Unicode by default.

[mhucka]

/gemini review

[gemini-code-assist]

Code Review

This pull request modernizes the codebase by removing legacy Python 2 compatibility artifacts. Key changes include transitioning to Python 3 class definitions by removing explicit inheritance from 'object', updating 'super()' calls to the simplified Python 3 syntax, and removing 'u' prefixes from string literals. Additionally, the pylint configuration and imports have been cleaned up to remove references to compatibility libraries like 'six' and 'future'. I have no feedback to provide.

[gemini-code-assist]

Code Review

This pull request modernizes the codebase by removing legacy Python 2 compatibility patterns. Key changes include removing explicit object inheritance from class definitions, updating super() calls to the simplified Python 3 syntax, and removing 'u' prefixes from string literals. Additionally, the Pylint configuration and utility imports were updated to remove references to Python 2/3 compatibility modules like six and builtins. I have no feedback to provide.