Generate an HTTP2 Request for piping to netcat

http2.py

#!/usr/bin/env python

from __future__ import print_function
import struct

HTTP2_HDR="PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"

# Does the thing for a frame
def frame(ty, flags, streamid, payload):
  return struct.pack(">L", len(payload))[1:4] + struct.pack(">BBL", ty, flags, streamid) + payload

last_stream_id = 0

# Returns a single naively hpacked value
def encode_header(key, value):
  key = key.lower()
  return struct.pack("<BB", 0, len(key)) + key + struct.pack("<B", len(value)) + value

# Does the thing for the HEADERS frame type
def headers(headers):
  global last_stream_id
  last_stream_id += 1
  # flags= (END_STREAM (0x1) | END_HEADERS (0x4))
  return frame(0x1, 0x5, last_stream_id, "".join(encode_header(x,y) for (x,y) in headers.items()))

def request(host, path, method="GET", scheme="http", extra_headers=None):
  h = {":authority": host, ":path": path, ":method": method, ":scheme": scheme}
  if extra_headers is not None:
    h.update(extra_headers)
  return headers(h)

print(HTTP2_HDR + frame(0x4, 0, 0, '') + request("localhost", "/example.php?foo=bar", extra_headers={"User-Agent": "baz"}), end="")

When I want to run this code I get this error:

Traceback (most recent call last):
  File "./pipeHttp2.py", line 32, in <module>
    print(HTTP2_HDR + frame(0x4, 0, 0, '') + request("localhost", "/example.php?foo=bar", extra_headers={"User-Agent": "baz"}), end="")
                      ^^^^^^^^^^^^^^^^^^^^
  File "./pipeHttp2.py", line 10, in frame
    return struct.pack(">L", len(payload))[1:4] + struct.pack(">BBL", ty, flags, streamid) + payload
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
TypeError: can't concat str to bytes