Make BZ2File.__init__()'s fileobj argument keyword-only.

Doc/library/bz2.rst

@@ -29,7 +29,7 @@ All of the classes in this module may safely be accessed from multiple threads.
 (De)compression of files
 ------------------------
 
-.. class:: BZ2File(filename=None, mode='r', buffering=None, compresslevel=9, fileobj=None)
+.. class:: BZ2File(filename=None, mode='r', buffering=None, compresslevel=9, \*, fileobj=None)
 
    Open a bzip2-compressed file.
 

Lib/bz2.py

@@ -40,7 +40,7 @@ class BZ2File(io.BufferedIOBase):
     """
 
     def __init__(self, filename=None, mode="r", buffering=None,
-                 compresslevel=9, fileobj=None):
+                 compresslevel=9, *, fileobj=None):
         """Open a bzip2-compressed file.
 
         If filename is given, open the named file. Otherwise, operate on