The flake8 tox environment is replaced with a "ruff" environment.

This environment runs both `ruff check` and `ruff format --check`.
Although ruff has the capability to fix both lint and format errors,
this tox env runs them such that they only report, but not fix, any
issues found.

The `select = ["E", "F"]` linter configuration corresponds to ruff's
default. The plan is to add more of ruffs linter rules incrementally,
while fixing associated issues in the SimPy codebase.