Make muc_nick return a const char* const

In the documentation of the `muc_nick` function we can read:

> The nickname is owned by the chat room and should not be modified or freed

We should reflect that in the return type and `strdup` the value for the
plugin API.

Fixes: https://github.com/profanity-im/profanity/issues/2013

src/plugins/python_api.c

@@ -435,7 +435,9 @@ python_api_get_current_nick(PyObject* self, PyObject* args)
     char* nick = api_get_current_nick();
     disable_python_threads();
     if (nick) {
-        return Py_BuildValue("s", nick);
+        PyObject* obj = Py_BuildValue("s", nick);
+        free(nick);
+        return obj;
     } else {
         Py_RETURN_NONE;
     }