Switch to unittest.mock from mock

Now that the minimum supported version of Python is 3.7, we can stop
using the external mock requirement, and import it from unittest. I have
also attempted to keep imports ordered.

Fixes #377

Author: s-t-e-v-e-n-k

noxfile.py

@@ -124,7 +124,6 @@ def default(session, install_grpc=True, prerelease=False, install_async_rest=Fal
 
     session.install(
         "dataclasses",
-        "mock",
         "pytest",
         "pytest-cov",
         "pytest-xdist",
@@ -280,7 +279,6 @@ def mypy(session):
         "types-setuptools",
         "types-requests",
         "types-protobuf",
-        "types-mock",
         "types-dataclasses",
     )
     session.run("mypy", "google", "tests")

tests/asyncio/future/test_async_future.py

@@ -13,8 +13,8 @@
 # limitations under the License.
 
 import asyncio
+from unittest import mock
 
-import mock
 import pytest
 
 from google.api_core import exceptions

tests/asyncio/gapic/test_method_async.py

@@ -13,8 +13,8 @@
 # limitations under the License.
 
 import datetime
+from unittest import mock
 
-import mock
 import pytest
 
 try:

tests/asyncio/operations_v1/test_operations_async_client.py

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
+
 import pytest
 
 try:

tests/asyncio/retry/test_retry_streaming_async.py

@@ -12,11 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import asyncio
 import datetime
 import re
-import asyncio
+from unittest import mock
 
-import mock
 import pytest
 
 from google.api_core import exceptions

tests/asyncio/retry/test_retry_unary_async.py

@@ -14,8 +14,8 @@
 
 import datetime
 import re
+from unittest import mock
 
-import mock
 import pytest
 
 from google.api_core import exceptions

tests/asyncio/test_grpc_helpers_async.py

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
+
 import pytest  # noqa: I202
 
 try:

tests/asyncio/test_operation_async.py

@@ -13,7 +13,8 @@
 # limitations under the License.
 
 
-import mock
+from unittest import mock
+
 import pytest
 
 try:

tests/asyncio/test_page_iterator_async.py

@@ -13,8 +13,8 @@
 # limitations under the License.
 
 import inspect
+from unittest import mock
 
-import mock
 import pytest
 
 from google.api_core import page_iterator_async

tests/asyncio/test_rest_streaming_async.py

@@ -15,14 +15,14 @@
 # TODO: set random.seed explicitly in each test function.
 # See related issue: https://github.com/googleapis/python-api-core/issues/689.
 
-import pytest  # noqa: I202
-import mock
-
 import datetime
 import logging
 import random
 import time
 from typing import List, AsyncIterator
+from unittest import mock
+
+import pytest  # noqa: I202
 
 import proto