tests: use stdlib mock instead of the third-party one

requirements-dev.txt

@@ -1,7 +1,6 @@
 -r requirements.txt
 -r requirements-optional.txt
 pytest>=3.2.5
-mock>=2.0.0
 flake8
 protobuf
 isort

setup.cfg

@@ -33,4 +33,4 @@ line_length = 88
 not_skip=__init__.py
 known_first_party=trezorlib
 known_third_party=hidapi, rlp, ethjsonrpc, ecdsa, mnemonic, requests, click, pyblake2, \
-    usb, construct, pytest, mock
+    usb, construct, pytest

trezorlib/tests/unit_tests/test_transport.py

@@ -14,7 +14,7 @@
 # You should have received a copy of the License along with this library.
 # If not, see <https://www.gnu.org/licenses/lgpl-3.0.html>.
 
-import mock
+from unittest import mock
 
 from trezorlib.transport import all_transports