demandimport: add 'nt' to ignore list (issue5373)

pathlib2 tries to import nt. Since it is a built-in module, there should be
no performance penalty.

https://github.com/mcmtroffaes/pathlib2/blob/release/2.2.0/pathlib2.py#L33

mercurial/demandimport.py

@@ -264,6 +264,7 b' ignore = ['
     '_imp',
     '_xmlplus',
     'fcntl',
+    'nt', # pathlib2 tests the existence of built-in 'nt' module
     'win32com.gen_py',
     '_winreg', # 2.7 mimetypes needs immediate ImportError
     'pythoncom',