Issue #20185: Convert _warnings.warn() to Argument Clinic

Fix warn_explicit(): interpret source=None as source=NULL.

Python/clinic/_warnings.c.h

@@ -0,0 +1,38 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(warnings_warn__doc__,
+"warn($module, /, message, category=None, stacklevel=1, source=None)\n"
+"--\n"
+"\n"
+"Issue a warning, or maybe ignore it or raise an exception.");
+
+#define WARNINGS_WARN_METHODDEF    \
+    {"warn", (PyCFunction)warnings_warn, METH_FASTCALL, warnings_warn__doc__},
+
+static PyObject *
+warnings_warn_impl(PyObject *module, PyObject *message, PyObject *category,
+                   Py_ssize_t stacklevel, PyObject *source);
+
+static PyObject *
+warnings_warn(PyObject *module, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"message", "category", "stacklevel", "source", NULL};
+    static _PyArg_Parser _parser = {"O|OnO:warn", _keywords, 0};
+    PyObject *message;
+    PyObject *category = Py_None;
+    Py_ssize_t stacklevel = 1;
+    PyObject *source = Py_None;
+
+    if (!_PyArg_ParseStack(args, nargs, kwnames, &_parser,
+        &message, &category, &stacklevel, &source)) {
+        goto exit;
+    }
+    return_value = warnings_warn_impl(module, message, category, stacklevel, source);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=b3c5297c2c55778c input=a9049054013a1b77]*/