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

Can confirm this behavior on CPython master as well. It seems that when an attribute is deleted then a deleted flag is set for the attribute at https://github.com/python/cpython/blob/73820a60cc3c990abb351540ca27bf7689bce8ac/Lib/unittest/mock.py#L737 . But when reset_mock is called it doesn't check for the deleted flag at https://github.com/python/cpython/blob/73820a60cc3c990abb351540ca27bf7689bce8ac/Lib/unittest/mock.py#L543

➜  cpython git:(master) ./python.exe ../backups/bpo31177.py
Traceback (most recent call last):
  File "../backups/bpo31177.py", line 5, in <module>
    m.reset_mock()
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 546, in reset_mock
    child.reset_mock(visited)
AttributeError: '_SentinelObject' object has no attribute 'reset_mock'

A simple patch would be to skip the deleted as below but some of the code in mock module raise an AttributeError. I don't know the correct behavior here. But applying the below patch and running tests with `./python.exe Lib/unittest/test/` doesn't cause any test failure.

diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index db1e642c00..700e2fb8b9 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -541,7 +541,7 @@ class NonCallableMock(Base):
             self._mock_side_effect = None
 
         for child in self._mock_children.values():
-            if isinstance(child, _SpecState):
+            if isinstance(child, _SpecState) or child is _deleted:
                 continue
             child.reset_mock(visited)
 
I will try to make a PR if it's ok.

Thanks

Adding Michael for thoughts on the fix and desired behavior. Removing 3.5 since only security fixes are accepted and adding 3.8 which is also affected.

Thanks

I suggest that after reset_mock(), deleted attributes should be available again. In other words, their deletion is reset.

I'm opening a PR to this effect.

I reported this issue to testing-cabal's mock repo in May 2016 (https://github.com/testing-cabal/mock/pull/361), but my original PR there just avoided an exception without reinstating the deleted attribute.

New changeset edeca92c84a3b08902ecdfe987cde00c7e617887 by Victor Stinner (Xtreak) in branch 'master':
bpo-31177: Skip deleted attributes while calling reset_mock (GH-9302)
https://github.com/python/cpython/commit/edeca92c84a3b08902ecdfe987cde00c7e617887

New changeset c0566e0ff6c2dd1a8b814ecd65649605c090527b by Miss Islington (bot) in branch '3.6':
bpo-31177: Skip deleted attributes while calling reset_mock (GH-9302)
https://github.com/python/cpython/commit/c0566e0ff6c2dd1a8b814ecd65649605c090527b

New changeset 422c1658b7d34fdc73c5fc895b135862103d1983 by Miss Islington (bot) in branch '3.7':
bpo-31177: Skip deleted attributes while calling reset_mock (GH-9302)
https://github.com/python/cpython/commit/422c1658b7d34fdc73c5fc895b135862103d1983

I am closing this as fixed since all the PRs were merged. Feel free to reopen this if needed. Thanks @mariocj89 and @vstinner for the review.