Fixed SyntaxWarning in py3.8
Fixed SyntaxWarning in py3.8

This prevents the following warning from appearing on Python 3.8:
```
/usr/local/lib/python3.8/site-packages/pusher/notification_client.py:45: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if len(interests) is 0:
```
pusher/pusher-http-python