Auto merge of #34597 - CensoredUsername:bootstrap-fix, r=alexcrichton

Support more python 2.7 versions in bootstrap.py

It seems python broke compatability between 2.7.9  and 2.7.12 as on the former a WindowsError was raised while on the latter a subprocess.CalledProcessError was raised while testing for the existence of uname.

As a WindowsError being thrown obviously indicates we're running on windows, this should probably be accepted too.