bpo-31238: pydoc ServerThread.stop() now joins itself (#3151)

* bpo-31238: pydoc ServerThread.stop() now joins itself

ServerThread.stop() now joins itself to wait until
DocServer.serve_until_quit() completes and then explicitly sets
its docserver attribute to None to break a reference cycle.

* Add NEWS.d entry
diff --git a/Lib/pydoc.py b/Lib/pydoc.py
index 376c445..5edc770 100644
--- a/Lib/pydoc.py
+++ b/Lib/pydoc.py
@@ -2297,6 +2297,10 @@
         def stop(self):
             """Stop the server and this thread nicely"""
             self.docserver.quit = True
+            self.join()
+            # explicitly break a reference cycle: DocServer.callback
+            # has indirectly a reference to ServerThread.
+            self.docserver = None
             self.serving = False
             self.url = None