Add Bach or Stravinsky helper

[alexanderthclark]

Summary

• Refresh PR Add Bach or Stravinsky helper #85 against current main with one clean commit.
• Add Game.bach_or_stravinsky() as a labeled constructor matching the Battle of the Sexes payoff structure.
• Preserve the current built-in constructor convention by setting player_names=("Anna", "Boris") and action_names=(("Bach", "Stravinsky"), ("Bach", "Stravinsky")).
• Add coverage for payoffs, player names, action names, and named Nash equilibria.

Testing

• pytest -q tests/test_games.py -> 18 passed, 1 warning
• pytest -q -> 134 passed, 2 warnings