Use the mock backport on Python 3.7

Because mock.call.kwargs, i.e. the ability to examine
m.call_args.kwargs where m is a Mock or MagicMock, was introduced
in Python 3.8.

Currently it is only in test/test_git.py that any use of mocks
requires this, so I've put the conditional import logic to import
mock (the top-level package) rather than unittest.mock only there.

The mock library is added as a development (testing) dependency
only when the Python version is lower than 3.8, so it is not
installed when not needed.

This fixes a problem in the new tests of whether a shell is used,
and reported as used, in the Popen call in Git.execute. Those
just-introduced tests need this, to be able to use
mock_popen.call_args.kwargs on Python 3.7.

test-requirements.txt

@@ -1,6 +1,7 @@
 black
 coverage[toml]
-ddt>=1.1.1, !=1.4.3
+ddt >= 1.1.1, != 1.4.3
+mock ; python_version < "3.8"
 mypy
 pre-commit
 pytest

test/test_git.py

@@ -13,7 +13,12 @@
 import subprocess
 import sys
 from tempfile import TemporaryDirectory, TemporaryFile
-from unittest import mock, skipUnless
+from unittest import skipUnless
+
+if sys.version_info >= (3, 8):
+    from unittest import mock
+else:
+    import mock  # To be able to examine call_args.kwargs on a mock.
 
 import ddt