Suggestions for helping contributors

[TG1999]

Some observations as a drive-by contributor, it would be lovely if:

• there was a pull request template
• the --diff settings in the CI linters told the whole story
  • maybe go to the all-in-one ruff (vs black + isort) which is a good deal simpler to reason about
• the mypy settings caught more things: files = src/packageurl/__init__.py isn't really finding the things a downstream would hit with the equivalent of mypy -p packageurl when used as a dependency

Originally posted by @bollwyvl in #213 (comment)