use annotations to express type hint returning self

NanoVNASaver/SITools.py

@@ -14,6 +14,7 @@
 #
 #  You should have received a copy of the GNU General Public License
 #  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+from __future__ import annotations
 import math
 import decimal
 from typing import NamedTuple, Union