Restrict type of AssignmentExpr.target to NameExpr

[bzoracler]

Assignment expression targets can only be identifiers. From the grammar:

assignment_expression:
    | NAME ':=' ~ expression 

This corresponds to the standard library AST node's target type:

class NamedExpr(expr):
    if sys.version_info >= (3, 10):
        __match_args__ = ("target", "value")
    target: Name

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅