implement workaround for \n{{ roundtripping bug

cf python/cpython#125008

mypy/fastparse.py

@@ -161,16 +161,14 @@ def p() -> AST:
         tokens = tokenize.generate_tokens(io.StringIO(source).readline)
     else:
         tokens = tokenize.tokenize(io.BytesIO(source).readline)
+    # We do a workaround for a roundtripping error https://github.com/python/cpython/issues/125008
+    # An error that was introduced in 3.12.3 (https://github.com/python/cpython/blob/v3.12.3/Lib/tokenize.py#L205) and fixed in 3.12.8 (not out yet at time of writing but it's probably https://github.com/python/cpython/blob/v3.12.8/Lib/tokenize.py#L203 or thereabouts).
+    # Luckily, it was caught before the first official (non-rc) release of python 3.13.
+    is_defective_version = (3, 12, 3) <= sys.version_info[:3] <= (3, 12, 7)
+    # Could the workaround ever come back to bite us? I confess I don't know enough about FSTRING_MIDDLE to say no for certain, even though I have tried to examine all relevant cases.
     source = tokenize.untokenize(
-        (
-            t,
-            (
-                re.sub(r"#\s*mypy:\s*ignore(?![-_])", "# type: ignore", s)
-                if t == tokenize.COMMENT
-                else s
-            ),
-        )
-        for t, s, *_ in tokens
+        (t, re.sub(r"#\s*mypy:\s*ignore(?![-_])", "# type: ignore", s) if t == tokenize.COMMENT else s+'{' if is_defective_version and t == tokenize.FSTRING_MIDDLE and s.startswith('\\') and s.endswith('{') else s)
+            for t, s, *_ in tokens
     )
     return p()