Don't depend on self.

[joshuria]

Remove self dependency in poetry pyproject.toml.

Without this, when poetry add, you get the following error:

$ poetry add pysimdjson
Using version ^7.0.1 for pysimdjson

Updating dependencies
Resolving dependencies... (0.0s) 
 
Package 'pysimdjson' is listed as a dependency of itself.