test_input_tty_non_ascii_unicode_errors() fails if readline was imported previously. Example:

$ ./python -m test test_readline test_builtin -v 
(...)
======================================================================
FAIL: test_input_tty_non_ascii (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2102, in test_input_tty_non_ascii
    self.check_input_tty("prompté", b"quux\xe9", "utf-8")
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2093, in check_input_tty
    self.assertEqual(input_result, expected)
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
?     +


======================================================================
FAIL: test_input_tty_non_ascii_unicode_errors (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2106, in test_input_tty_non_ascii_unicode_errors
    self.check_input_tty("prompté", b"quux\xe9", "ascii")
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2093, in check_input_tty
    self.assertEqual(input_result, expected)
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
?     +



Another example: import readline in test_builtin.

diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index 40df7b606a..102b397e66 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -1,3 +1,4 @@
+import readline
 # Python test set -- built-in functions
 
 import ast


$ ./python -m test test_builtin -v -m PtyTests
== CPython 3.10.0a0 (heads/master:310f6aa7db, Jun 19 2020, 12:18:54) [GCC 10.1.1 20200507 (Red Hat 10.1.1-1)]
== Linux-5.6.18-300.fc32.x86_64-x86_64-with-glibc2.31 little-endian
== cwd: /home/vstinner/python/master/build/test_python_123487
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
0:00:00 load avg: 0.36 Run tests sequentially
0:00:00 load avg: 0.36 [1/1] test_builtin
test_input_no_stdout_fileno (test.test_builtin.PtyTests) ... ok
test_input_tty (test.test_builtin.PtyTests) ... ok
test_input_tty_non_ascii (test.test_builtin.PtyTests) ... FAIL
test_input_tty_non_ascii_unicode_errors (test.test_builtin.PtyTests) ... FAIL

======================================================================
FAIL: test_input_tty_non_ascii (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2102, in test_input_tty_non_ascii
    self.check_input_tty("prompté", b"quux\xe9", "utf-8")
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2093, in check_input_tty
    self.assertEqual(input_result, expected)
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
?     +


======================================================================
FAIL: test_input_tty_non_ascii_unicode_errors (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2106, in test_input_tty_non_ascii_unicode_errors
    self.check_input_tty("prompté", b"quux\xe9", "ascii")
  File "/home/vstinner/python/master/Lib/test/test_builtin.py", line 2093, in check_input_tty
    self.assertEqual(input_result, expected)
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
?     +


----------------------------------------------------------------------

Ran 4 tests in 0.028s

FAILED (failures=2)
test test_builtin failed
test_builtin failed

== Tests result: FAILURE ==

1 test failed:
    test_builtin

Total duration: 446 ms
Tests result: FAILURE