bpo-8138: Initialize wsgiref's SimpleServer as single-threaded (GH-12977)

Lib/wsgiref/simple_server.py

@@ -127,7 +127,8 @@ class WSGIRequestHandler(BaseHTTPRequestHandler):
             return
 
         handler = ServerHandler(
-            self.rfile, self.wfile, self.get_stderr(), self.get_environ()
+            self.rfile, self.wfile, self.get_stderr(), self.get_environ(),
+            multithread=False,
         )
         handler.request_handler = self      # backpointer for logging
         handler.run(self.server.get_app())

Misc/NEWS.d/next/Library/2019-04-27-02-54-23.bpo-8138.osBRGI.rst

+Don't mark ``wsgiref.simple_server.SimpleServer`` as multi-threaded since
+``wsgiref.simple_server.WSGIServer`` is single-threaded.