Extra dependency `asyncio` included

[setu4993]

asyncio is a dependency only for upto Python 3.4. Since the versions of Python supported are 3.7+, it doesn't seem to be required, but is included.

This is causing issues with running pip check in conda-forge/twilio-feedstock#94, which I help maintain.

Technical details:

• twilio-python version: 8.0.0
• python version: 3.7+

[nelsonlazar]

asyncio has broken our Serverless Lambda function(Python v3.8):

[ERROR] SyntaxError: invalid syntax (base_events.py, line 296)
...
File "/var/task/django/utils/deprecation.py", line 1, in
import asyncio
File "/var/task/asyncio/init.py", line 21, in
from .base_events import *

As a workaround fixed using version lock with previous version of Twilio

[kridai]

Thanks for the PR. The changes have been merged, and would reflect in the next twilio-python release.