Monkey patching support

These are just some not terribly well though out ideas about possible monkey support in typing. This would need a proper PEP if it was ever implemented, but I just want to record my ideas here:

# foo accepts a date object, but allows to add and access
# unknown attributes at runtime. It basically treats date
# as having __getattr__()/__setattr__() methods.
def foo(date: Dynamic[datetime.date]) -> None:
    date.my_attr = 123
    print(date.added_by_called)


# typing.dynamic() is a no-op at runtime and basically an alias
# for cast(Generic[X], y) at type-check time, where X is the
# type of y.
# In this example, dt has type Dynamic[datetime.date].
dt = dynamic(datetime.date.today())
foo(dt)

# The following would also work:
foo(datetime.date.today())
other_dt: datetime.date = dt

A possible extensions would be to define the allowed monkey-patched attributes. Something like:

class Monkey(Protocol):
    donkey: int

def foo(dt: Dynamic[datetime.date, Monkey]) -> None:
    # ok:
    print(dt.year)
    dt.donkey += 3
    # errors:
    print(dt.kong)
    dt.donkey += ""

dt = dynamic(datetime.date.today(), Monkey)

首先审阅提议的 Dynamic 和 dynamic 示例，包括基于 Protocol 的允许属性扩展，然后阅读包含七条评论的讨论，以了解其中的约束或异议。此 issue 旨在探索，而不是已经可以实施；要完成它，需要先达成一致的设计，并完成 issue 中提到的适当 PEP，之后才能开始代码工作。