gh-128646: Implement GzipFile.readinto[1]() methods (GH-128647)

gh-128646: Implement GzipFile.readinto[1]() methods (GH-128647)

The new methods simply delegate to the underlying buffer, much like the existing GzipFile.read[1] methods.  This avoids extra allocations caused by the BufferedIOBase.readinto implementation previously used.

This commit also factors out a common readability check rather than copying it an additional two times.