Prefer Python3 for the remotely-executed Python script

This should be done as part of our general transition to Python3,
but should also be done because some systems with e.g. Python2.6
do not support OrderedDict, but DO have a Python3 version that works.

pwnlib/tubes/ssh.py

@@ -932,6 +932,14 @@ def func(): pass
 
         func_src  = inspect.getsource(func).strip()
         setuid = True if setuid is None else bool(setuid)
+
+        # executable may be a unicode string on Python2 and will get a u-prefix
+        # when passed to repr() in the script below.  
+        #
+        # This breaks things if a Python3 interpreter is selected.
+        executable = six.ensure_str(executable)
+        argv = [bytes(a) for a in argv]
+        env = None if not env else { six.ensure_str(k): six.ensure_str(v) for k,v in env}
 
         script = r"""
 #!/usr/bin/env python
@@ -942,7 +950,7 @@ def func(): pass
 except NameError:
     integer_types = int,
 exe   = %(executable)r
-argv  = [bytes(a) for a in %(argv)r]
+argv  = %(argv)r
 env   = %(env)r
 
 os.chdir(%(cwd)r)
@@ -1076,7 +1084,7 @@ def is_exe(path):
 
         with self.progress(msg) as h:
 
-            script = 'for py in python2.7 python2 python; do test -x "$(which $py 2>&1)" && exec $py -c %s check; done; echo 2' % sh_string(script)
+            script = 'for py in python3 python2.7 python2 python; do test -x "$(which $py 2>&1)" && exec $py -c %s check; done; echo 2' % sh_string(script)
             with context.quiet:
                 python = ssh_process(self, script, tty=True, raw=True, level=self.level, timeout=timeout)