You must run this test as administrator.

C:\Users\vajrasky\Code\cpython>PCbuild\python.exe Lib\test\test_pathlib.py
..........s..s..s..s.........s.....E............ssssssssssssssssssssssssssssssss
ssssssssssssssssssssssssssssssssssssssssssssssss................................
................................................................................
...............................................s..s..s..s.........s.....E.......
..
======================================================================
ERROR: test_resolve_dot (__main__.PathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib\test\test_pathlib.py", line 1281, in test_resolve_dot
    self.assertEqual(q.resolve(), p)
  File "C:\Users\vajrasky\Code\cpython\lib\pathlib.py", line 1017, in resolve
    s = self._flavour.resolve(self)
  File "C:\Users\vajrasky\Code\cpython\lib\pathlib.py", line 179, in resolve
    return self._ext_to_normal(_getfinalpathname(s))
OSError: [WinError 123] The filename, directory name, or volume label syntax is
incorrect: 'C:\\Users\\vajrasky\\Code\\cpython\\@test_5860_tmp\\2'

======================================================================
ERROR: test_resolve_dot (__main__.WindowsPathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib\test\test_pathlib.py", line 1281, in test_resolve_dot
    self.assertEqual(q.resolve(), p)
  File "C:\Users\vajrasky\Code\cpython\lib\pathlib.py", line 1017, in resolve
    s = self._flavour.resolve(self)
  File "C:\Users\vajrasky\Code\cpython\lib\pathlib.py", line 179, in resolve
    return self._ext_to_normal(_getfinalpathname(s))
OSError: [WinError 123] The filename, directory name, or volume label syntax is
incorrect: 'C:\\Users\\vajrasky\\Code\\cpython\\@test_5860_tmp\\2'

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

Windows does not like "/" in its name for the file.

Attached the patch to fix the test.

Wow, thank you. It's a pity none of the Windows buildbots runs in administrator mode.

New changeset 076824de7650 by Antoine Pitrou in branch 'default':
Issue #19877: fix regression in test_pathlib when Windows has symlink support available (i.e. in administrator mode).
http://hg.python.org/cpython/rev/076824de7650

Now committed.

My build slave ran as admin in order to make sure symlinks were covered, but I don't have the hardware anymore. I'll see if I can get another machine up and running.