Virtualenv 20.x is not supported

[jesse-peters]

I had to downgrade to virtualenv 16.7.10 to get this to work, otherwise I get the following error when I attempt a sls package:

Serverless: [pyIndividually] Installing packagings: python testproject/lib/_requirements.py testproject/requirements.txt testproject/lib
Serverless: [pyIndividually] Traceback (most recent call last):
  File "testproject/lib/_requirements.py", line 57, in <module>
    virtualenv.main()

serverless.yml

service: testproject
frameworkVersion: "2.11.1"

package:
    individually: True
    exclude:
        - "**/*"
plugins:
    - serverless-python-individually

provider:
    name: aws
    runtime: python3.6
    region: us-west-2

functions:
    testproject:
        handler: testproject/wrap.handler
        package:
            include:
                - testproject/**
custom:
    pyIndividually:
        wrap:testproject: testproject/runner.handle_message

[paulchaum]

I made a PR to fix this error : #22