Apply suggestions from code review

This appeases mypy (once the version is upgraded in the pre-commit config
file) and removes some superfluous bits.

Thanks @bmerry for the suggestions.

Co-authored-by: Bruce Merry <1963944+bmerry@users.noreply.github.com>

src/aiokatcp/adjtimex.py

@@ -134,10 +134,11 @@ def _errcheck(result, func, args):
 except OSError:
     adjtimex = _no_adjtimex
 else:
-    adjtimex = _libc.adjtimex
-    adjtimex.argtypes = [ctypes.POINTER(Timex)]  # type: ignore[attr-defined]
-    adjtimex.restype = ctypes.c_int  # type: ignore[attr-defined]
-    adjtimex.errcheck = _errcheck  # type: ignore[attr-defined]
+    _real_adjtimex = _libc.adjtimex
+    _real_adjtimex.argtypes = [ctypes.POINTER(Timex)]
+    _real_adjtimex.restype = ctypes.c_int
+    _real_adjtimex.errcheck = _errcheck
+    adjtimex = _real_adjtimex
 
 
 def get_adjtimex() -> Tuple[int, Timex]:

tests/test_time_sync.py

@@ -67,17 +67,17 @@ def mock_adjtimex(mocker, request) -> None:
 
 
 _linux = pytest.mark.skipif(
-    not sys.platform.startswith("linux"),
+    sys.platform != "linux",
     reason="Check real adjtimex on Linux systems only",
 )
 _non_linux = pytest.mark.skipif(
-    sys.platform.startswith("linux"),
+    sys.platform == "linux",
     reason="Check unimplemented adjtimex on non-Linux systems only",
 )
 
 
 @pytest.mark.parametrize(
-    ("expected_status",),
+    "expected_status",
     [
         pytest.param(Sensor.Status.NOMINAL, marks=_linux),
         pytest.param(Sensor.Status.INACTIVE, marks=_non_linux),