Fix import causing integration failures on Windows

There is limited to no pty support on Windows, so test cases calling it are
skipped on Windows. This PR changes test behavior to only import pty on non-NT
platforms.

test/integration/dss/test_dss_cli.py

@@ -4,7 +4,6 @@
 import filecmp
 import json
 import os
-import pty
 import sys
 import unittest
 import uuid
@@ -131,6 +130,7 @@ def test_upload_progress_bar(self):
     @unittest.skipIf(os.name is 'nt', 'No pty support on Windows')
     def test_upload_progress_bar_interactive(self):
         """Tests upload progress bar with a simulated interactive session"""
+        import pty  # Trying to import this on Windows will cause a ModuleNotFoundError
         dirpath = os.path.join(TEST_DIR, 'tutorial', 'data')  # arbitrary and small
         put_args = ['dss', 'upload', '--src-dir', dirpath, '--replica',
                     'aws', '--staging-bucket', 'org-humancellatlas-dss-cli-test']