Test syntax error on comma-less tuple-style sequence patterns (#115485)

Adds a test that length-1 tuple-style sequence patterns must end in a comma, since there isn't currently one.

Spotted while reviewing Cython's proposed implementation of the pattern matching syntax (https://github.com/cython/cython/pull/4897#discussion_r1489177169) where there was a bug my the reimplementation that wasn't caught against the CPython tests here.