Fix USDT probes arguments' encoding in Python3 (iovisor#1736)

* Fix USDT probes arguments' encoding in Python3

Running `trace` on a binary's USDT while fetching some arguments (
`sudo python3 trace.py -p $(pidof ruby) 'u:ruby:array__create "%d",
arg1'`) fails with `argument 2: <class 'TypeError'>: wrong type`.

This PR fixes the encoding of the USDT probe name in
udst.py `get_probe_arg_ctype` function. I've tested this works on Python 2 too.

src/python/bcc/usdt.py

@@ -175,7 +175,7 @@ def get_text(self):
 
  def get_probe_arg_ctype(self, probe_name, arg_index):
  return lib.bcc_usdt_get_probe_argctype(
- self.context, probe_name, arg_index)
+ self.context, probe_name.encode('ascii'), arg_index).decode()
 
  def enumerate_probes(self):
  probes = []