replace old "mock" with newer "unittest.mock" from the standard library

requirements.txt

@@ -18,7 +18,6 @@ hypothesis>=3.27.0,<6.72.2
 pytest>=8.2.0
 pytest-cov>=2.7.0,<5.0.0
 coverage>=5.0.0,<8.0.0
-mock>=2.0.0,<3.0.0
 flake8>=3.5.0,<3.6.0
 mypy>=1.5.0,<1.6.0; platform_machine != "aarch64"
 types-mock>=0.1.1

thinc/tests/layers/test_linear.py

@@ -1,7 +1,8 @@
+from unittest.mock import MagicMock
+
 import numpy
 import pytest
 from hypothesis import given, settings
-from mock import MagicMock
 from numpy.testing import assert_allclose
 
 from thinc.api import SGD, Dropout, Linear, chain

thinc/tests/layers/test_with_debug.py

@@ -1,4 +1,4 @@
-from mock import MagicMock
+from unittest.mock import MagicMock
 
 from thinc.api import Linear, with_debug