Schema fields with dict attribute names return dict attributes when keys missing

Whenever a Schema is defined that has fields that shadow dict attributes (e.g. "items", "keys", "values", etc), passing a dict with those keys not present to Schema.dump() results in dict attributes being serialized:

from marshmallow import Schema, fields

class A(Schema):
    a = fields.Str()
    keys = fields.Str(default='k')
    values = fields.Str(missing='v')
    items = fields.Str()

# No problem on load.
loaded = A().load({'a': 'x'})
print(loaded.data)
# {'a': 'x', 'values': 'v'}

# No problem when fields present.
dumped = A().dump({'a': 'x', 'keys': 'k', 'values': 'v', 'items': 'i'})
print(dumped.data)
# {'a': 'x', 'keys': 'k', 'items': 'i', 'values': 'v'}

# Undesired attribute access when fields not present.
dumped = A().dump({'a': 'x'})
print(dumped.data)

# on marshmallow==2.13.5
# {'a': 'x',
#  'keys': "dict_keys(['a'])",
#  'values': "dict_values(['x'])",
#  'items': "dict_items([('a', 'x')])"}

# on marshmallow==3.0.0b2
# {'a': 'x',
#  'keys': '<built-in method keys of dict object at 0x7f4e206bc948>'}
#  'values': '<built-in method values of dict object at 0x7f4e206bc948>',
#  'items': '<built-in method items of dict object at 0x7f4e206bc948>',

I found #30 which indicates that this is intended behavior for dumping dict objects, but I was wondering if 3.0.0 may be open to changing it (for dict classes only) or providing an option to disable dict-attribute access when dumping.

I'm aware that I could change the attribute name in the Schema definition and utilize load_from/dump_to but I wanted to avoid aliasing to keep the Schema attributes the same as the de-/serialization format.

In my own application, I've overridden Schema.get_attribute with a custom implementation so that dict attributes aren't accessed at all but it would be nice to not to have to do this in each project.

If overriding Schema.get_attribute is the preferred method for implementing this functionality (instead of having a first-class option/setting), then I'll keep doing that. However, it may be worth adding a warning or note in the documentation about this "gotcha" when dumping a dict object with a Schema that mirrors dict attributes. The section on Specifying Attribute Names does mention object attribute access but might be helpful to call out the potential field/dict-attribute conflicts that can result when dumping (unless I'm missing something else in the docs that covers this scenario).

Thoughts?

Start by reviewing Schema.get_attribute and the documentation section on Specifying Attribute Names referenced in the issue. Compare the dict-attribute behavior for missing fields with the intended marshmallow 3 behavior, then identify whether the resolved scope is a code change, an option, or documentation; done requires an agreed behavior and corresponding coverage or documentation.