Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Modules/clinic/_datetimemodule.inc

@@ -0,0 +1,40 @@
+/* clang-format off */
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(datetime_datetime_now__doc__,
+"now($type, /, tz=None)\n"
+"--\n"
+"\n"
+"Returns new datetime object representing current time local to tz.\n"
+"\n"
+"  tz\n"
+"    Timezone object.\n"
+"\n"
+"If no tz is specified, uses local timezone.");
+
+#define DATETIME_DATETIME_NOW_METHODDEF    \
+    {"now", (PyCFunction)datetime_datetime_now, METH_FASTCALL|METH_CLASS, datetime_datetime_now__doc__},
+
+static PyObject *
+datetime_datetime_now_impl(PyTypeObject *type, PyObject *tz);
+
+static PyObject *
+datetime_datetime_now(PyTypeObject *type, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"tz", NULL};
+    static _PyArg_Parser _parser = {"|O:now", _keywords, 0};
+    PyObject *tz = Py_None;
+
+    if (!_PyArg_ParseStack(args, nargs, kwnames, &_parser,
+        &tz)) {
+        goto exit;
+    }
+    return_value = datetime_datetime_now_impl(type, tz);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=8aaac0705add61ca input=a9049054013a1b77]*/