Older versions of python 2.7 can raise a WindowsError instead of a su…

…bprocess.CalledProcessError when `uname` does not exist

src/bootstrap/bootstrap.py

@@ -265,7 +265,7 @@ def build_triple(self):
         try:
             ostype = subprocess.check_output(['uname', '-s']).strip()
             cputype = subprocess.check_output(['uname', '-m']).strip()
-        except subprocess.CalledProcessError:
+        except (subprocess.CalledProcessError, WindowsError):
             if sys.platform == 'win32':
                 return 'x86_64-pc-windows-msvc'
             err = "uname not found"