bootstrap: add python2 error check

Fail hard when run under Python 2. This catches delegated commands as
well, since those are run by this file.

Signed-off-by: Marti Bolivar <marti@foundries.io>

src/bootstrap/main.py

@@ -13,6 +13,9 @@ import platform
 import subprocess
 import sys
 
+if sys.version_info < (3,):
+    sys.exit('fatal error: you are running Python 2')
+
 MANIFEST = 'manifest'
 MANIFEST_DEFAULT = 'https://github.com/zephyrproject-rtos/manifest'
 MANIFEST_REV_DEFAULT = 'master'