fix mypy linting: python/mypy#9242

Author: winsonluk

src/pip/_internal/commands/install.py

@@ -227,11 +227,11 @@ def add_options(self):
     @with_cleanup
     def run(self, options, args):
         # type: (Values, List[str]) -> int
-        try:                                                     # Unix
+        if sys.platform == 'darwin' or sys.platform == 'linux':  # Unix
             is_su = os.getuid() == 0                             # type: ignore
-        except AttributeError:                                   # Windows
+        elif sys.platform == 'win32' or sys.platform == 'cygwin':  # Windows
             is_su = ctypes.windll.shell32.IsUserAnAdmin() != 0   # type: ignore
-        except Exception:                                        # Unknown
+        except AttributeError:                                   # Unknown
             is_su = False
         if is_su:
             sudo_warning = ("Running pip as root is not recommended. "