Is this still reproducible? On master (Python 3.8) with a debug build it throws a SyntaxError. I don't have Python 3.5 installed to check this though

$ ./python.exe
Python 3.8.0a0 (heads/master:c87d9f406b, Sep 23 2018, 19:48:30)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
➜  cpython git:(master) ./python.exe -c 'with open("vuln.py", "wb") as f: f.write(b"\x61\x73\x00\x0a\x79\x6e\x63\x5c\x0a\xef")'
➜  cpython git:(master) ✗ ./python.exe vuln.py
  File "vuln.py", line 2
SyntaxError: Non-UTF-8 code starting with '\xef' in file vuln.py on line 2, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details
➜  cpython git:(master) ✗ ./python.exe -c 'with open("vuln2.py", "wb") as f: f.write(b"\x61\x73\x00\x0a\x79\x6e\x63\x5c\x0a\x00\x0d\xdd")'
➜  cpython git:(master) ✗ ./python.exe vuln2.py
  File "vuln2.py", line 3
SyntaxError: Non-UTF-8 code starting with '\xdd' in file vuln2.py on line 3, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details


Thanks