Fix System Error on Python 3.13 and Windows (#457)

Fixes #456.

The relevant crash from the test log actually seems to be the initial
failure:

```
AttributeError: module 'time' has no attribute 'clock_gettime_ns'

The above exception was the direct cause of the following exception:

...

>       _time_machine.patch_if_needed()
E       SystemError: <built-in function patch_if_needed> returned a result with an exception set
```

`PyObject_GetAttrString` fails to get `clock_gettime` and
`clock_gettime_ns` on Windows because they only exist on Unix. It
returns `NULL` and each time sets the `AttributeError` as the current
error. `patch_if_needed` did not clear this, leading to Python's call
check to raise a `SystemError` caused by the `AttributeError`, making
time-machine fail to start travelling.

I'm not sure which exact change in Python made this failure occur, so
far it seems the `SystemError` should have been raised on older versions
too. Ah well.

CHANGELOG.rst

@@ -2,6 +2,10 @@
 Changelog
 =========
 
+* Fix ``SystemError`` on Python 3.13 and Windows when starting time travelling.
+
+  Thanks to Bernát Gábor for the report in `Issue #456 <https://github.com/adamchainz/time-machine/issues/456>`__.
+
 2.14.1 (2024-03-22)
 -------------------
 

src/_time_machine.c

@@ -423,21 +423,25 @@ _time_machine_patch_if_needed(PyObject *module, PyObject *unused)
 
     PyObject *time_module = PyImport_ImportModule("time");
 
-
-
-    PyCFunctionObject *time_clock_gettime = (PyCFunctionObject *) PyObject_GetAttrString(time_module, "clock_gettime");
     /*
-        time.clock_gettime() is not always available
-        e.g. on builds against old macOS = official Python.org installer
+        time.clock_gettime(), only available on Unix platforms.
     */
-    if (time_clock_gettime != NULL) {
+    PyCFunctionObject *time_clock_gettime = (PyCFunctionObject *) PyObject_GetAttrString(time_module, "clock_gettime");
+    if (time_clock_gettime == NULL) {
+        PyErr_Clear();
+    } else {
         state->original_clock_gettime = time_clock_gettime->m_ml->ml_meth;
         time_clock_gettime->m_ml->ml_meth = _time_machine_clock_gettime;
         Py_DECREF(time_clock_gettime);
     }
 
+    /*
+        time.clock_gettime_ns(), only available on Unix platforms.
+    */
     PyCFunctionObject *time_clock_gettime_ns = (PyCFunctionObject *) PyObject_GetAttrString(time_module, "clock_gettime_ns");
-    if (time_clock_gettime_ns != NULL) {
+    if (time_clock_gettime_ns == NULL) {
+        PyErr_Clear();
+    } else {
         state->original_clock_gettime_ns = time_clock_gettime_ns->m_ml->ml_meth;
         time_clock_gettime_ns->m_ml->ml_meth = _time_machine_clock_gettime_ns;
         Py_DECREF(time_clock_gettime_ns);