Fix issue iovisor#1744. (iovisor#1746)

- Fix a crash in the python binding when trying to open a perf buffer
    in python < 3.6
- See iovisor#1744

src/python/bcc/__init__.py

@@ -445,12 +445,12 @@ def get_table(self, name, keytype=None, leaftype=None, reducer=None):
  if map_fd < 0:
  raise KeyError
  if not keytype:
- key_desc = lib.bpf_table_key_desc(self.module, name)
+ key_desc = lib.bpf_table_key_desc(self.module, name).decode("utf-8")
  if not key_desc:
  raise Exception("Failed to load BPF Table %s key desc" % name)
  keytype = BPF._decode_table_type(json.loads(key_desc))
  if not leaftype:
- leaf_desc = lib.bpf_table_leaf_desc(self.module, name)
+ leaf_desc = lib.bpf_table_leaf_desc(self.module, name).decode("utf-8")
  if not leaf_desc:
  raise Exception("Failed to load BPF Table %s leaf desc" % name)
  leaftype = BPF._decode_table_type(json.loads(leaf_desc))