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.

Lib/idlelib/pyshell.py

@@ -635,6 +635,9 @@ class ModifiedInterpreter(InteractiveInterpreter):
         if source is None:
             with tokenize.open(filename) as fp:
                 source = fp.read()
+                if use_subprocess:
+                    source = (f"__file__ = r'''{os.path.abspath(filename)}'''\n"
+                              + source + "\ndel __file__")
         try:
             code = compile(source, filename, "exec")
         except (OverflowError, SyntaxError):