fix instance down counting

Dominik Neise · Dominik Neise · commit 6e8ba590d501 · 2018-04-30T12:01:20.000+02:00
diff --git a/protozfits/simple.py b/protozfits/simple.py
@@ -40,7 +40,7 @@ def __exit__(self, type, value, tb):
         self.close()
 
     def close(self):
-        File.instances = 0
+        File.instances -= 1
 
     def __del__(self):
         self.close()
