How to get hashlib to work on both Python2 and Python3

Question

This code works fine when using Python2, however when using Python3 it gives me this error:

Checksum error: bob.tgz, Unicode-objects must be encoded before hashing

I then made some changes to use byte like objects in the sha256 object but then I started getting:

Checksum error: bob.tgz, 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

So my question is how do I get this code to work on both Python2 and Python3?

import hashlib

filepath = "bob.tgz"
filepath_hashed = "bob.tgz.hashed"
thekey = "asdf1234"
try:
    m = hashlib.sha256()

    file_contents = None
    with open(filepath, "r") as f_in:
        file_contents = f_in.read()
        m.update(file_contents)
    m.update(thekey)
    with open(filepath_hashed, "w") as f_out:
        f_out.write(file_contents)
        f_out.write("~~CHECKSUM~~%s" % m.hexdigest())
except Exception as e:
    print("Checksum error: %s, %s" % (filepath, e))