attr.ib cleanup + auto_attribs + class vars from superclasses

[hynek]

Consider the following:

import attr

class Base:
    x = 0

@attr.s(auto_attribs=True)
class C(Base):
    x: int

This raises an:

Traceback (most recent call last):
  File "t.py", line 9, in <module>
    class C(Base):
  File "/Users/hynek/Projects/attrs/src/attr/_make.py", line 858, in wrap
    return builder.build_class()
  File "/Users/hynek/Projects/attrs/src/attr/_make.py", line 496, in build_class
    return self._patch_original_class()
  File "/Users/hynek/Projects/attrs/src/attr/_make.py", line 514, in _patch_original_class
    delattr(cls, name)
AttributeError: x

(interestingly getattr(cls, name) works but delattr does not)

This is actually quite common if you implement an ABC. The current code is:

                if (
                    name not in super_names
                    and getattr(cls, name, None) is not None
                ):
                    delattr(cls, name)

Maybe we should change the check to isinstance(getattr(cls, name, None), _CountingAttr)?

[hynek]

(Maybe wrapping in try/except would be better.)