fix ResourceWarning when writing a unwritable gzipfile

Author: graingert

Lib/gzip.py

@@ -245,8 +245,13 @@ def __init__(self, filename=None, mode=None,
 
         self.fileobj = fileobj
 
-        if self.mode == WRITE:
-            self._write_gzip_header(compresslevel)
+        try:
+            if self.mode == WRITE:
+                self._write_gzip_header(compresslevel)
+        except BaseException:
+            # Avoid a ResourceWarning if the write fails, eg read-only file or KI
+            self.fileobj = None
+            raise
 
     @property
     def mtime(self):