Running test -v test_winconsoleio showed that the hang is in test_cntl_z.

With that commented out, I get failures in test_input (1st assert) and test_partial_reads and some bizarre console behavior.

F:\dev\3x>python -m test -v test_winconsoleio
Running Debug|Win32 interpreter...
== CPython 3.10.0a0 (heads/master:1264d0465a, Sep 5 2020, 18:53:05) [MSC v.1900 32 bit (Intel)]
== Windows-10-10.0.19041-SP0 little-endian
== cwd: F:\dev\3x\build\test_python_7960æ
== CPU count: 12
== encodings: locale=cp1252, FS=utf-8
0:00:00 Run tests sequentially
0:00:00 [1/1] test_winconsoleio
test_abc (test.test_winconsoleio.WindowsConsoleIOTests) ... ok
test_conin_conout_names (test.test_winconsoleio.WindowsConsoleIOTests) ... ok
test_conout_path (test.test_winconsoleio.WindowsConsoleIOTests) ... ok
test_input (test.test_winconsoleio.WindowsConsoleIOTests) ... aaaaaaaaaabbbbbbbbbbcccccccccc111111111122222222223333333333
FAIL
test_input_nonbmp (test.test_winconsoleio.WindowsConsoleIOTests) ... skipped 'Handling Non-BMP characters is broken'
test_open_fd (test.test_winconsoleio.WindowsConsoleIOTests) ... ok
test_open_name (test.test_winconsoleio.WindowsConsoleIOTests) ... ok
test_partial_reads (test.test_winconsoleio.WindowsConsoleIOTests) ...
FAIL
test_partial_surrogate_reads (test.test_winconsoleio.WindowsConsoleIOTests) ... skipped 'Handling Non-BMP characters is broken'
test_write_empty_data (test.test_winconsoleio.WindowsConsoleIOTests) ... ok

======================================================================
FAIL: test_input (test.test_winconsoleio.WindowsConsoleIOTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "f:\dev\3x\lib\test\test_winconsoleio.py", line 142, in test_input
    self.assertStdinRoundTrip('abc123')
  File "f:\dev\3x\lib\test\test_winconsoleio.py", line 138, in assertStdinRoundTrip
    self.assertEqual(actual, text)
AssertionError: 'aaaaaaaaaabbbbbbbbbbcccccccccc111111111122222222223333333333' != 'abc123'
- aaaaaaaaaabbbbbbbbbbcccccccccc111111111122222222223333333333
+ abc123


======================================================================
FAIL: test_partial_reads (test.test_winconsoleio.WindowsConsoleIOTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "f:\dev\3x\lib\test\test_winconsoleio.py", line 168, in test_partial_reads
    self.assertEqual(actual, expected, 'stdin.read({})'.format(read_count))
AssertionError: b'\r\n' != b'\xcf\xbc\xd1\x9e\xd0\xa2\xce\xbb\xd0\xa4\xd0\x99\r\n' : stdin.read(1)

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

Ran 10 tests in 0.019s

FAILED (failures=2, skipped=2)
test test_winconsoleio failed
test_winconsoleio failed

== Tests result: FAILURE ==

1 test failed:
    test_winconsoleio

Total duration: 266 ms
Tests result: FAILURE

F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>ϼϼϼϼϼϼϼϼϼϼўўўўўўўўўўТТТТТТТТТТλλλλλλλλλλФФФФФФФФФФЙЙЙЙЙЙЙЙЙЙ
'ϼϼϼϼϼϼϼϼϼϼўўўўўўўўўўТТТТТТТТТТλλλλλλλλλλФФФФФФФФФФЙЙЙЙЙЙЙЙЙЙ' is not recognized as an internal or external command,
operable program or batch file.

F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>
F:\dev\3x>

Even though behavior is the same in 3 different workspaces, and the file shows no obvious sign of corruption in Notepad++, I deleted it anyway (in master) with Windows Explorer 'delete' (not git delete)  and then reverted the change.  Still, someone should verify that this is not just my machine.

Note that I just got the 2008 updates (last Tuesday?) to Win 10, 2004, some of which were apparently optional at the moment.