Remove pathname from runtime_dependency

[soutaro]

I found that installing pathname into non default location with bundle config path ??? caused a problem, because pathname is not vendored in bundler.

You have already activated pathname 0.1.0, but your Gemfile requires pathname 0.2.1. Since pathname is a default gem, you can either remove your dependency on it or try updating to a newer version of bundler that supports pathname as a default gem. (Gem::LoadError)

https://github.com/soutaro/bundler-pathname-error/blob/main/test

So, removing dependency from gemspec of Steep would be a quick solution.