Here is a unittest where you can test this issue and the result on Python 3.8.0a2 and 3.7.2

>>> 3.8.0a2
./python /tmp/test_bug_36338.py
/tmp/test_bug_36338.py:8: SyntaxWarning: invalid escape sequence \[
  url = 'http://demo.com\[attacker.com]'
3.8.0a2+ (heads/master:23581c018f, Mar 18 2019, 09:17:05) 
[GCC 8.3.1 20190223 (Red Hat 8.3.1-2)]
test_bad_url (__main__.TestUrlparse) ... FAIL

======================================================================
FAIL: test_bad_url (__main__.TestUrlparse)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/test_bug_36338.py", line 13, in test_bad_url
    self.assertEqual(hostname, expected_hostname)
AssertionError: 'attacker.com' != 'demo.com'
- attacker.com
+ demo.com


----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

>>> 3.7.2
python /tmp/test_bug_36338.py
3.7.2 (default, Jan 16 2019, 19:49:22) 
[GCC 8.2.1 20181215 (Red Hat 8.2.1-6)]
test_bad_url (__main__.TestUrlparse) ... FAIL

======================================================================
FAIL: test_bad_url (__main__.TestUrlparse)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/test_bug_36338.py", line 13, in test_bad_url
    self.assertEqual(hostname, expected_hostname)
AssertionError: 'attacker.com' != 'demo.com'
- attacker.com
+ demo.com


----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)