Switch from pipenv to poetry

- dropped dev dependency on rope, isort & flake
- poetry doesn't support dev scripts like pipenv, so create a makefile instead
- Add pytest-cov
- Use tox for testing multiple python versions in CI
- Update README

Update ci workflow

.gitignore

@@ -1,3 +1,5 @@
+.coverage
+.DS_Store
 .env
 .vscode/settings.json
 .mypy_cache
@@ -9,4 +11,5 @@ betterproto/tests/output_*
 dist
 **/*.egg-info
 output
-.idea
+.idea
+.tox