Tweak code-style and readability in `events/base.py`

Tweak code-style and readability in `events/base.py`

This includes a few low-key, syntax-level tweaks and:

1. Rewrite of the if-statment in `Events._accept_with`.
2. Property name change, i.e. from `dispatcher.dispatch_cls` to
`dispatcher.dispatch`. In this case postfix `_cls` is confusing as
the property is not a class, but an instance of one.