rewrite Stackelberg-eq solver in cvxpy; add degenerated test case

[rezunli96]

Just found the original lp solver using cvxopt may not handle the degenerated case very well. Rewrote them in cvxpy.

[lanctot]

Problem on MacOS-12:

24/262 Test #239: python/algorithms/stackelberg_lp_test.py ..................***Failed    3.25 sec
Traceback (most recent call last):
  File "/Users/runner/work/open_spiel/open_spiel/open_spiel/python/algorithms/stackelberg_lp_test.py", line 36, in <module>
    game2 = pyspiel.create_matrix_game([[3, 9], [9, 1],
TypeError: create_matrix_game(): incompatible function arguments. The following argument types are supported:
    1. (arg0: str, arg1: str, arg2: list[str], arg3: list[str], arg4: list[list[float]], arg5: list[list[float]]) -> pyspiel.MatrixGame
    2. (arg0: list[list[float]], arg1: list[list[float]]) -> pyspiel.MatrixGame
    3. (arg0: list[list[float]], arg1: list[list[float]]) -> pyspiel.MatrixGame

[lanctot]

You need the payoffs for the other player(s).

(Or, if it's a two-player zero-sum game, feel free to add a create_two_player_zero_sum_matrix_game that takes only the first player's payoffs)

[lanctot]

Test failed; add a numerical tolerance