Declare Python 3.13 support

Python 3.13 was added to tested runtimes during this cycle.

Change-Id: I130f250467e9bbf9125f87a1ab7efa093daae546
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
diff --git a/setup.cfg b/setup.cfg
index bc52823..af8c903 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -21,6 +21,7 @@
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
     Programming Language :: Python :: 3.12
+    Programming Language :: Python :: 3.13
     Topic :: Internet :: Name Service (DNS)
 
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