dummy_thread.acquire() would return None if no waitflag argument was given. It
should have returned True.

Fixes issue #3339. Thanks, Henk Punt for the report and Andrii v. Mishkovskiyi
for attempting a patch.
diff --git a/Lib/dummy_thread.py b/Lib/dummy_thread.py
index c131384..16dcf7e 100644
--- a/Lib/dummy_thread.py
+++ b/Lib/dummy_thread.py
@@ -104,18 +104,15 @@
         aren't triggered and throw a little fit.
 
         """
-        if waitflag is None:
+        if waitflag is None or waitflag:
             self.locked_status = True
-            return None
-        elif not waitflag:
+            return True
+        else:
             if not self.locked_status:
                 self.locked_status = True
                 return True
             else:
                 return False
-        else:
-            self.locked_status = True
-            return True
 
     __enter__ = acquire