make the re.sub take str or bytes, like the function signature

Author: wyattscarpenter

mypy/fastparse.py

@@ -140,7 +140,10 @@ def ast3_parse(
 ) -> AST:
     # Hack to support "mypy: ignore" comments until the builtin compile function changes to allow us to detect it otherwise:
     # (does not apply at the start of the line to avoid conflicting with mypy file configuration comments https://mypy.readthedocs.io/en/stable/inline_config.html ; see also, util.get_mypy_comments in this codebase)
-    source = re.sub(r"(?<!^)#\s*mypy:\s*ignore", "# type: ignore", source)
+    if isinstance(source, str):
+        source = re.sub(r"(?<!^)#\s*mypy:\s*ignore", "# type: ignore", source)
+    else:
+        source = re.sub(rb"(?<!^)#\s*mypy:\s*ignore", b"# type: ignore", source)
     return ast3.parse(
         source,
         filename,