bpo-32984: IDLE - set __file__ for startup files (GH-5981)

bpo-32984: IDLE - set __file__ for startup files (GH-5981)

Like Python, IDLE optionally runs one startup file in the Shell window
before presenting the first interactive input prompt.  For IDLE,
option -s runs a file named in environmental variable IDLESTARTUP or
PYTHONSTARTUP; -r file runs file.  Python sets __file__ to the startup
file name before running the file and unsets it before the first
prompt.  IDLE now does the same when run normally, without the -n
option.
(cherry picked from commit 22c82be5df70c3d51e3f89b54fe1d4fb84728c1e)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>