async_mixin.py

class AsyncMixin:
  async def async_get(self, url):
    return "Async download: "+url

client.py

import sys

if sys.version_info >= (3,5,2):
  # Not executed on old Python, no syntax error
  from async_mixin import AsyncMixin
else:
  class AsyncMixin:
    pass
  
class Client(AsyncMixin):
  def get(self, url):
    return "Sync download: "+url

main.py

import asyncio
from client import Client

client = Client()

# Works anytime
content = client.get("urn:test")
# Works only if 3.5.2 at least
import asyncio
loop = asyncio.get_event_loop()
loop.run_until_complete(client.async_get("urn:test"))