Fix return type in get_linux_type() (#1025)

Summary:
This function is expected to return a tuple of 3 elements. This doesn't happen when it fails opening the `/etc/os-release` file.

X-link: https://github.com/facebook/watchman/pull/1025

Reviewed By: fanzeyi

Differential Revision: D36736667

Pulled By: chadaustin

fbshipit-source-id: a6839235d6c5ebb50267cd78f1c9590b2fb180d7

build/fbcode_builder/getdeps/platform.py

@@ -7,6 +7,7 @@ import platform
 import re
 import shlex
 import sys
+from typing import Optional, Tuple
 
 
 def is_windows() -> bool:
@@ -15,12 +16,12 @@ def is_windows() -> bool:
     return sys.platform.startswith("win")
 
 
-def get_linux_type():
+def get_linux_type() -> Tuple[Optional[str], Optional[str], Optional[str]]:
     try:
         with open("/etc/os-release") as f:
             data = f.read()
     except EnvironmentError:
-        return (None, None)
+        return (None, None, None)
 
     os_vars = {}
     for line in data.splitlines():