[3.12] gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (GH-118320) (GH-118691)

gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (GH-118320)

Fix an edge case in `binascii.a2b_base64` strict mode, where
excessive padding was not detected when no padding is necessary.

(cherry picked from commit fe47d9bee319528ffeb5fd60a615d7f02c7b5585)

Co-authored-by: Youfu Zhang <1315097+zhangyoufu@users.noreply.github.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>