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Not very long unicode comment #, space and then 170 or more repetitions of the utf8 symbol ░ (b'\xe2\x96\x91'.decode()) 
# ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
causes syntax error:
SyntaxError: Non-UTF-8 code starting with '\xe2' in file tst112.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details
Python file is attached. Second example is similar, but here unicode string with similar length is used as an argument of a print function.
print('\n░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░')
Similar Issue34979 was submitted one year ago...

I think that this should be closed as a duplicate of #34979 and this example posted there, with the OS and python version included.
On Windows, with 3.7, 3.8.0, and master, neither the posted comment, the one in the file, not the initial statement in #34979 give the SyntaxError.

> On Windows, with 3.7, 3.8.0, and master, neither the posted comment, the one in the file, not the initial statement in #34979 give the SyntaxError.
Just tried again on my corporate laptop with the downloaded file from this site:
Microsoft Windows [Version 10.0.16299.1451]
(c) 2017 Microsoft Corporation. All rights reserved.
D:\Downloads>py
Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> quit()
D:\Downloads>py tst112.py
 File "tst112.py", line 1
SyntaxError: Non-UTF-8 code starting with '\xe2' in file tst112.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details
d:\Downloads>py -3.7
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul 8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> quit()
d:\Downloads>py -3.7 tst112.py
 File "tst112.py", line 1
SyntaxError: Non-UTF-8 code starting with '\xe2' in file tst112.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details

Just tested again:
D:\Downloads>py 
Python 3.9.4 (tags/v3.9.4:1f2e308, Apr 4 2021, 13:27:16) [MSC v.1928 64 bit (AMD64)] on win32 
Type "help", "copyright", "credits" or"license" for more information. 
>>> quit()
 D:\Downloads>py tst112.py 
SyntaxError: Non-UTF-8 code starting with '\xe2' in file D:\Downloads\tst112.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details 
P.S. No problems with Python 3.8.5 and Ubuntu 20.04.2 LTS.

> P.S. No problems with Python 3.8.5 and Ubuntu 20.04.2 LTS.
The issue is that the line length is limited to BUFSIZ, which ends up splitting the UTF-8 sequence b'\xe2\x96\x91'. BUFSIZ is only 512 bytes in Windows. It's 8192 bytes in Linux, in which case you need a line that's 16 times longer in order to reproduce the error. For example:
 $ stat -c "%s" test.py 
 8194
 $ python3.9 test.py
 SyntaxError: Non-UTF-8 code starting with '\xe2' in file 
 /home/someone/test.py on line 1, but no encoding declared; see 
 http://python.org/dev/peps/pep-0263/ for details
This has been fixed in a rewrite of the tokenizer (bpo-25643), for which the PR was recently merged into the main branch for 3.10a7+.
Maybe a minimal backport to keep reading up to "\n" can be applied to 3.8 and 3.9.

The bpo-14811 issue was fixed in Python 3.10 by bpo-25643, but is not fixed in Python 3.8 and 3.9.

In 2012, I wrote detect_truncate.patch in bpo-14811. Does someone want to convert it to a PR for Python 3.9?