Change a test, because it doesn't really matter

The test had changed behavior for Python 3.8, a syntax error of:

SyntaxError: unexpected EOF while parsing

instead of

SyntaxError: invalid syntax

test/failing_examples.py

@@ -146,7 +146,7 @@ def build_nested(code, depth, base='def f():\n'):
     # Now nested parsing
     "f'{continue}'",
     "f'{1;1}'",
-    "f'{a*=3}'",
+    "f'{a;}'",
     "f'{b\"\" \"\"}'",
 ]