Add minimal support for pattern matching

[kennipj]

Pattern matching syntax is currently unsupported in the current release of black, and will result in black crashing.

This PR attempts to provide minimal support for pattern matching, supporting most syntax, but more advanced use cases may not be covered.

Also supports soft-keyword use of match and case, as specified in PEP634.

I can't guarantee how well black will format code of long match/case statements, but it shouldn't break in most use cases.

No tests have been added here, as this is out of scope for this PR. Purpose is purely to make a quick fix to make black support pattern matching.

Some examples of syntax that is supported with this PR:

match point:
    case Point(x, y) if x == y:
        print(f"Y=X at {x}")
    case Point(x, y):
        print(f"Not on the diagonal")

match command.split():
    case ["go", direction] if direction in current_room.exits:
        current_room = current_room.neighbor(direction)
    case ["go", _]:
        print("Sorry, you can't go that way")

match command.split():
    case ["drop", *objects]:
        for obj in objects:
            character.drop(obj, current_room)

    match command.split():
        case ["north"] | ["go", "north"]:
            current_room = current_room.neighbor("north")
        case ["get", obj] | ["pick", "up", obj] | ["pick", obj, "up"]:
            ...  # Code for picking up the given object

Example of syntax not supported:

    match command.split():
        case ["go", ("north" | "south" | "east" | "west") as direction]:
            current_room = current_room.neighbor(direction)

This will fail on case ["go", ("north" | "south" | "east" | "west") as direction]:.

[povilasb]

Lets make a new branch though (argyle?), so that we could always pull latest black main branch and rebase off it if needed ;)