suppress_thread_exceptions doesn't fail under Python 3.6.

It uses threading.excepthook, which doesn't exist in pre-3.8 Python. But there's no need to break all the tests under 3.6.

suppress_thread_exceptions was introduced in commit 001698cd.

kepi/sombrero_sendpub/tests/__init__.py

@@ -7,9 +7,25 @@ def suppress_thread_exceptions():
     Context manager which causes exceptions in threads not to
     be printed. See https://stackoverflow.com/questions/63206653/ .
     """
-    orig = threading.excepthook
-    threading.excepthook = lambda a: None
     try:
-        yield
-    finally:
-        threading.excepthook = orig
+        orig = threading.excepthook
+        threading.excepthook = lambda a: None
+        try:
+            yield
+        finally:
+            threading.excepthook = orig
+    except AttributeError:
+
+        # Python 3.6 doesn't have threading.excepthook,
+        # but everything else in kepi works. So if it's
+        # missing, we work around that.
+
+        print("""Warning:
+        You are running a version of Python without threading.excepthook.
+        You will see spurious error messages during testing.
+        """)
+
+        try:
+            yield
+        finally:
+            pass