Upgrade PyYaml to 6.0

[TaylorMcGinnis]

Hi there @michaelboulton
Would it be possible to upgrade the dependency to use PyYAML 6.0? I'm facing some dependency conflicts between Tavern and another package I rely on heavily (Seleniumbase) as I am wanting to move to Selenium 4.0+. Seleniumbase requires at least PyYAML 6.0 in order to use the earliest version that it supports Selenium 4.0. Ideally, Pytest 7.1.1 could also be used in Tavern so I can use the latest version of their package (I know Pytest 7.0 was mentioned in this other issue)