Fix Windows man pages guard (os.system to sys.platform)

httpie/output/ui/man_pages.py

@@ -1,7 +1,8 @@
 """Logic for checking and displaying man pages."""
 
-import subprocess
 import os
+import subprocess
+import sys
 from httpie.context import Environment
 
 
@@ -18,7 +19,7 @@ def is_available(program: str) -> bool:
     Check whether `program`'s man pages are available on this system.
 
     """
-    if NO_MAN_PAGES or os.system == 'nt':
+    if NO_MAN_PAGES or sys.platform == 'win32':
         return False
     try:
         process = subprocess.run(

tests/test_man_pages.py

@@ -0,0 +1,12 @@
+from unittest.mock import Mock
+
+from httpie.output.ui import man_pages
+
+
+def test_man_pages_are_unavailable_on_windows(monkeypatch):
+    run = Mock()
+    monkeypatch.setattr(man_pages.sys, 'platform', 'win32')
+    monkeypatch.setattr(man_pages.subprocess, 'run', run)
+
+    assert man_pages.is_available('http') is False
+    run.assert_not_called()