FIX: Hack around 3.12rc1 bug (python/cpython#108111)

Author: effigies

nibabel/openers.py

@@ -78,6 +78,12 @@ def __init__(
             mtime=mtime,
         )
 
+    def seek(self, pos: int, whence: int = 0, /) -> int:
+        # Work around bug (gh-180111) in Python 3.12rc1, where seeking without
+        # flushing can cause write of excess null bytes
+        self.flush()
+        return super().seek(pos, whence)
+
 
 def _gzip_open(
     filename: str,