Fix path separator issue in change_root function

use `os.sep` instead of hardcoding `\\`
also, fix appropriate tests

Author: naveen521kk

distutils/tests/test_util.py

@@ -109,6 +109,7 @@ def _join(*path):
 
         # windows
         os.name = 'nt'
+        os.sep = '\\'
 
         def _isabs(path):
             return path.startswith('c:\\')

distutils/util.py

@@ -165,7 +165,7 @@ def change_root(new_root, pathname):
 
     elif os.name == 'nt':
         (drive, path) = os.path.splitdrive(pathname)
-        if path[0] == '\\':
+        if path[0] == os.sep:
             path = path[1:]
         return os.path.join(new_root, path)