bpo-31234: Join threads in test_hashlib (#3573)

* bpo-31234: Join threads in test_hashlib

Use thread.join() to wait until the parallel hash tasks complete
rather than using events. Calling thread.join() prevent "dangling
thread" warnings.

* test_hashlib: minor PEP 8 coding style fixes

Lib/test/test_hashlib.py

@@ -746,28 +746,28 @@ class HashLibTestCase(unittest.TestCase):
         hasher = hashlib.sha1()
         num_threads = 5
         smallest_data = b'swineflu'
-        data = smallest_data*200000
+        data = smallest_data * 200000
         expected_hash = hashlib.sha1(data*num_threads).hexdigest()
 
-        def hash_in_chunks(chunk_size, event):
+        def hash_in_chunks(chunk_size):
             index = 0
             while index < len(data):
-                hasher.update(data[index:index+chunk_size])
+                hasher.update(data[index:index + chunk_size])
                 index += chunk_size
-            event.set()
 
-        events = []
+        threads = []
         for threadnum in range(num_threads):
-            chunk_size = len(data) // (10**threadnum)
+            chunk_size = len(data) // (10 ** threadnum)
             self.assertGreater(chunk_size, 0)
             self.assertEqual(chunk_size % len(smallest_data), 0)
-            event = threading.Event()
-            events.append(event)
-            threading.Thread(target=hash_in_chunks,
-                             args=(chunk_size, event)).start()
+            thread = threading.Thread(target=hash_in_chunks,
+                                      args=(chunk_size,))
+            threads.append(thread)
 
-        for event in events:
-            event.wait()
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
 
         self.assertEqual(expected_hash, hasher.hexdigest())