[test suite] grammar fuzz failure with utf-8

[charles-cooper]

test failure (from https://github.com/vyperlang/vyper/actions/runs/8343830170/job/22834860255?pr=3861#step:6:88)
=========================== short test summary info ============================
FAILED tests/functional/grammar/test_grammar.py::test_grammar_bruteforce - vyper.exceptions.InvalidLiteral: 'ā' is not an allowed string literal character

line 2:0
1 """"""
---> 2 """ā"""
-------^
3

Falsifying example: test_grammar_bruteforce(
code='"""""" \n"""ā"""',
)

You can reproduce this example by temporarily adding @reproduce_failure('6.99.9', b'AXicY+RjAAFGRkYozQABAAGzABU=') as a decorator on your test case
============ 1 failed, 737 passed, 16 warnings in 697.42s (0:11:37) ============
Error: Process completed with exit code 1.