bpo-37824: Properly handle user input warnings in IDLE shell. (GH-15500)

Cease turning SyntaxWarnings into SyntaxErrors.
(cherry picked from commit 1039f39c9c6edb4c185856c19316d3a4eb561c38)

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