Generate seeds every time a board is generated

[seeing-hands]

This change is designed to generate fresh random seeds whenever a Sudoku object is instantiated. In the original version, if a seed is not explicitly provided, it is generated at import time instead of at instantiation time. This means that repeated calls to the constructor without a seed generate the same board like this:

>>> import sudoku
>>> b1 = sudoku.Sudoku(3, 3)
>>> b2 = sudoku.Sudoku(3, 3)
>>> b1.board == b2.board
True
>>> s1 = sudoku.Sudoku(3, 3).solve()
>>> b1.solve().board == s1.board
True

This modification generates a fresh seed every time the constructor is called if the user does not provide their own.