Properly check for invalid conversion character with f-string debugging syntax (#156)

test/failing_examples.py

@@ -357,3 +357,7 @@ if sys.version_info[:2] >= (3, 8):
         '(None := 1)',
         '(__debug__ := 1)',
     ]
+    # f-string debugging syntax with invalid conversion character
+    FAILING_EXAMPLES += [
+        "f'{1=!b}'",
+    ]

test/test_fstring.py

@@ -79,6 +79,7 @@ def test_valid(code, grammar):
 
         # invalid conversion characters
         'f"{1!{a}}"',
+        'f"{1=!{a}}"',
         'f"{!{a}}"',
 
         # The curly braces must contain an expression