fix Windows short-circuit in man_pages.is_available

is_available guarded against Windows with `os.system == 'nt'`. os.system
is the built-in function object from the os module, never a string, so
the comparison was always False and the function never short-circuited
on Windows even though that was clearly the author's intent (the
surrounding code is checking whether the running OS has the `man`
binary). On Windows the function fell through to `subprocess.run([MAN_COMMAND, ...])`,
which is missing from PATH and raised FileNotFoundError that was then
swallowed by the bare `except Exception`, so users saw a confusing
'no man pages found' instead of an immediate 'not on Windows' early
return.

Author: Zo Bot

httpie/output/ui/man_pages.py

@@ -18,7 +18,7 @@ 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 os.name == 'nt':
         return False
     try:
         process = subprocess.run(