Add a warning (a mistake was made on py3.6, but the warning applies here too)

Author: arigo

pypy/goal/targetpypystandalone.py

@@ -126,6 +126,7 @@ def pypy_setup_home(ll_home, verbose):
             space.appexec([w_path, space.newtext(home), w_initstdio],
             r"""(path, home, initstdio):
                 import sys
+                # don't import anything more above this: sys.path is not set
                 sys.path[:] = path
                 sys.executable = home
                 initstdio(unbuffered=True)