Abstract readable/writable properties support

[sobolevn]

This code is not considered valid by mypy:

from abc import abstractmethod

class Array(object):
    @property
    @abstractmethod
    def _length_(self) -> int: ...

    @_length_.setter
    @abstractmethod
    def _length_(self, value: int) -> None: ...

Output:

out/ex.py:4: error: Overloaded method has both abstract and non-abstract variants
out/ex.py:7: error: Decorated property not supported

But, this code is valid:

1. It works in runtime
2. It is documented here: https://docs.python.org/3/library/abc.html#abc.abstractproperty
3. It has clear semantics: we need subclasses of this type to have a read/write property implementation

Refs python/typeshed#6361

[AlexWaygood]

This is #1362, no?

[sobolevn]

It is related, but we have one more extra warning: Overloaded method has both abstract and non-abstract variants

[AlexWaygood]

Closing as a duplicate of #4165