gh-130806: Emit ResourceWarning if GzipFile unclosed (#130905)

This may indicate accidental data loss.

Ways to make sure all data is written:
1. Use the [file-like object](https://docs.python.org/3/glossary.html#term-file-object) as a [“With Statement Context Manager”](https://docs.python.org/3/reference/datamodel.html#context-managers).
   - All objects which [inherit](https://docs.python.org/3/tutorial/classes.html#inheritance) from [IOBase](https://docs.python.org/3/library/io.html#io.IOBase) support this.
   - [`BufferedIOBase`](https://docs.python.org/3/library/io.html#io.BufferedIOBase), [`BufferedWriter`](https://docs.python.org/3/library/io.html#io.BufferedWriter), and [`GzipFile`](https://docs.python.org/3/library/gzip.html#gzip.GzipFile) all support this.
   - Ensures `.close()` is called in both exception and regular cases.

2. Ensure [`.close()`](https://docs.python.org/3/library/io.html#io.IOBase.close) is always called which flushes data before closing.

3. If the underlying stream need to be kept open, use [`.detach()`](https://docs.python.org/3/library/io.html#io.BufferedIOBase.detach)

Since 3.12 flushing has been necessary in GzipFile (see gh-105808 which was a release blocker), this makes that more visible. Users have been encountering as they upgrade to 3.12 (ex. gh-129726).

There are a number of cases of unclosed file-like objects being deleted in CPython libraries and the test suite. This issue includes resolving those cases where the new ResourceWarning is emitted.

Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/gzip.py

@@ -193,6 +193,11 @@ class GzipFile(_compression.BaseStream):
 
         """
 
+        # Ensure attributes exist at __del__
+        self.mode = None
+        self.fileobj = None
+        self._buffer = None
+
         if mode and ('t' in mode or 'U' in mode):
             raise ValueError("Invalid mode: {!r}".format(mode))
         if mode and 'b' not in mode:
@@ -368,7 +373,9 @@ class GzipFile(_compression.BaseStream):
 
     def close(self):
         fileobj = self.fileobj
-        if fileobj is None or self._buffer.closed:
+        if fileobj is None:
+            return
+        if self._buffer is None or self._buffer.closed:
             return
         try:
             if self.mode == WRITE:
@@ -445,6 +452,13 @@ class GzipFile(_compression.BaseStream):
         self._check_not_closed()
         return self._buffer.readline(size)
 
+    def __del__(self):
+        if self.mode == WRITE and not self.closed:
+            import warnings
+            warnings.warn("unclosed GzipFile",
+                          ResourceWarning, source=self, stacklevel=2)
+
+        super().__del__()
 
 def _read_exact(fp, n):
     '''Read exactly *n* bytes from `fp`