Issue #6378: Further adjust idle.bat to start associated Python

Also, make sure that test_zlib tests decompress() for overly-large inputs.

attribute when called without a max_length argument.

attribute when called without a max_length argument.

a smarter "py-bt" command printing a classic Python traceback.

on a file opened in read+write mode (namely: reading, seeking a bit forward,
writing, then seeking before the previous write but still within buffered
data, and writing again).

on a file opened in read+write mode (namely: reading, seeking a bit forward,
writing, then seeking before the previous write but still within buffered
data, and writing again).

_tkinter.c is returning <class '_tkinter.Tcl_Obj'> sometimes.  Don't use
tkinter.messagebox.Message - use the helper functions which convert to str.

With Tk < 8.5 _tkinter.c:PythonCmd() raised UnicodeDecodeError, caused IDLE to
exit.  Converted to valid Unicode null in PythonCmd().

bytes. The PEP-0333 says that and test_wsgiref follows the same.  Updated docs
accordingly.