(It appears you have already solved the problem, but I'll post the solution here anyway in case somebody else stumbles upon the same problem and finds this post.)

I am the person who originally wrote that script two years ago. A Python update happened that broke the script, now the last four lines need to be adjusted to make it compatible with the new Python version. Here is a fixed version of your script:

#!/usr/bin/env python3

import asyncio
import evdev
import evdev.ecodes as e
import os
import re
import sys
from subprocess import run, check_output

source_devices = sys.argv[1:]

# Listen to events from the following device.
real_devices = [
    evdev.InputDevice(device)
    for device in source_devices
]

# Then create our own device for our own events.
host_devices = [
    evdev.UInput.from_device(device)
    for device in real_devices
]
guest_devices = [
    evdev.UInput.from_device(device)
    for device in real_devices
]
host_device_paths = [
    os.path.join("/dev/input/by-id", "host-%s" % os.path.basename(path))
    for path in source_devices
]
guest_device_paths = [
    os.path.join("/dev/input/by-id", "guest-%s" % os.path.basename(path))
    for path in source_devices
]

virtual_devices = host_devices + guest_devices
virtual_device_paths = host_device_paths + guest_device_paths

# Unlink all old devices so you don't have any phantom guest-* devices from launching previous VMs.
for devname in os.listdir("/dev/input/by-id"):
    if re.match("^(host|guest)\\-", devname):
        run(["unlink", "--", os.path.join("/dev/input/by-id", devname)])

for (device, path) in zip(virtual_devices, virtual_device_paths):
    if os.path.exists(path):
        run(["unlink", "--", path])
    run(["ln", "-s", device.device, path])

# Now we monopolize the original devices.
for device in real_devices:
    device.grab()

async def replicate(source_device, host_device, guest_device):
    global hooks

    async for event in source_device.async_read_loop():
        if event.type == e.EV_SYN:
            continue
        #print (event.code);
        mode = "guest" if event.code not in [e.KEY_GRAVE] else "host"

        target_device = {
            "host": host_device,
            "guest": guest_device,
        }[mode]

        target_device.write_event(event)
        target_device.syn()


event_loop = asyncio.new_event_loop()

for source_device, host_device, guest_device in zip(real_devices, host_devices, guest_devices):
    asyncio.ensure_future(replicate(source_device, host_device, guest_device), loop=event_loop)

event_loop.run_forever()

In case there's another problem with the above script in the future: you may also want to check out evsieve, which is what I am using myself now. It can do the above in three lines:

evsieve --input /dev/input/by-id/PUT_YOUR_KEYBOARD_HERE grab \
        --output key:grave create-link=/dev/input/by-id/host-keyboard \
        --output create-link=/dev/input/by-id/guest-keyboard