[issue31177] unittest mock's reset_mock throws an error when an attribute has been deleted

[Hmvp]

New submission from Hmvp:

When using a mock and deleting a attribute reset_mock cannot be used anymore since it tries to call reset_mock on the _deleted sentinel value.

Reproduction path:
```
from unittest.mock import MagicMock
mock = MagicMock()
mock.a = 'test'
del mock.a
mock.reset_mock()
```

Gives:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/unittest/mock.py", line 544, in reset_mock
    child.reset_mock(visited)
AttributeError: '_SentinelObject' object has no attribute 'reset_mock'
```

Expected result:
mock is reset without throwing an exception and the 'a' attribute is no longer in a deleted state

Only checked 3.5 and current master if bug is present

----------
components: Library (Lib)
messages: 300090
nosy: hmvp
priority: normal
severity: normal
status: open
title: unittest mock's reset_mock throws an error when an attribute has been deleted
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31177>
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