You should use

ascii_term_id.decode()

instead of

str(ascii_term_id)

Code

print( b'0500'.decode() )
print( str(b'0500') )

on Python 2 gives

0500
0500

but on Python 3 gives

0500
b'0500'

EDIT:

You need only

print('ascii_term_id? "' + ascii_term_id.decode() + '"')

because decode() converts bytes to string (Python3) and you don't need str() any more.

ascii_term_id = b'0500'

print( type( ascii_term_id ) )
print( type( ascii_term_id.decode() ) )

print('ascii_term_id? "' + ascii_term_id.decode() + '"')

Python 2:

<type 'str'>
<type 'unicode'>
ascii_term_id? "0500"

Python 3

<class 'bytes'>
<class 'str'>
ascii_term_id? "0500"