Separate and simplify PyDis specific usage

[jb3]

This PR makes a start at decoupling the specific setup of snekbox PyDis uses and snekbox as a standalone application.

This is done by building a basic snekbox application that supports evaluation using only the system version of Python.

We then have a new separate Dockerfile.pydis that uses the pre-built versions from python-discord/python-builds.

An integration specific image using this Dockerfile is built for integration tests to mirror the current behaviour.

[coveralls]

coverage: 88.265%. remained the same when pulling d08f0bc73117f89f78784cec674ae6239ea19fc9 on jb3/pydis-version-separation into 28c95c87c4ae7e036e23c963bc55e3eccebaec93 on main.