Prefer unittest.mock before mock

[mimi1vx]

From Python 3.3 is mock part of python standard library in unittest namespace

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[RonnyPfannschmidt]

good reminder, needing mock is insane - good tests should do without

[coveralls]

Coverage increased (+0.05%) to 92.677% when pulling deff54a on mimi1vx:patch-1 into 50a0d4f on pytest-dev:master.

[asottile]

there are occasions where this is undesirable, for instance I'd prefer to use the backported mock (>=2) over the stdlib in python3.4:

$ python3.4 -c 'from unittest import mock; mock.Mock().assert_wat()'
$ python3.4 -c 'import mock; mock.Mock().assert_wat()'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python3.4/dist-packages/mock/mock.py", line 703, in __getattr__
    raise AttributeError(name)
AttributeError: assert_wat

[mimi1vx]

ok so first try import mock and with ImportError stdlib..

[asottile]

I'm actually more of a +0/-0 on this change -- this is part of the test suite for pytest and not part of the public package. pytest also hard codes a test dependency on mock so the import should never fail under normal circumstances

[nicoddemus]

I'm with @asottile, but @RonnyPfannschmidt seems to be +1 about it.

[mimi1vx]

@asottile @nicoddemus so I changed logic to try import first upstream mock and if this fail stdlib unittest.mock