Knight pairs

You are given a chess board of $8 \times 8$ squares. Each square is either empty or contains a knight. In the description of the board, the character . means an empty square and the character * means a knight.

Your task is to form as many pairs of knights as possible. The knights in a pair must attack each other, and each knight can belong to at most one pair. Two knights attack each other if either their horizontal or their vertical distance is $1$ and the other distance is $2$ (as in the rules of chess).

In a file knights.py, implement the function count_pairs that returns the maximum number of pairs.

def count_pairs(grid):
    # TODO

if __name__ == "__main__":
    grid = ["*.......",
            "..*...*.",
            "........",
            ".*......",
            "...*....",
            ".......*",
            "........",
            "......*."]

    print(count_pairs(grid)) # 3

    grid = ["*.***.**",
            "*******.",
            "*.**.**.",
            "***.****",
            "..****.*",
            "********",
            "*.******",
            "***.**.*"]

    print(count_pairs(grid)) # 25