gh-138514: getpass: restrict `echo_char` to a single ASCII character (#138591)

This amends commit bf8bbe9a813dd9fc2dd14be06df172b7d26ca1af by
restricting `echo_char` in `getpass.getpass` to single printable
ASCII characters as it would be uncommon to use long strings or
multi-byte characters for keyboard feedback.

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Brian Schubert <brianm.schubert@gmail.com>