Add PyPy3 to Travis test suite (#5193)

.travis.yml

@@ -15,7 +15,10 @@ jobs:
     - stage: test
       python: '3.6'
     - stage: test
-      python: '3.7'
+      python: '3.7' 
+    - stage: test
+      python: 'pypy3'
+      dist: xenial
     - stage: test
       python: '3.8'
     - stage: coverage

tests/testserver/server.py

@@ -96,6 +96,7 @@ class Server(threading.Thread):
             handler_result = self.handler(sock)
 
             self.handler_results.append(handler_result)
+            sock.close()
 
     def _accept_connection(self):
         try: