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

Makefile

@@ -0,0 +1,43 @@
+help:               ##	- Show this help.
+	@fgrep -h "##" $(MAKEFILE_LIST) | fgrep -v fgrep | sed -e 's/\\$$//' | sed -e 's/##//'
+
+# Dev workflow tasks
+
+setup:              ##	- Setup the virtualenv with poetry
+	poetry install -E compiler
+
+generate:           ##	- Generate test cases (do this once before running test)
+	poetry run ./betterproto/tests/generate.py
+
+test:               ##	- Run tests
+	poetry run pytest --cov betterproto
+
+types:              ##	- Check types with mypy
+	poetry run mypy betterproto --ignore-missing-imports
+
+format:             ##	- Apply black formatting to source code
+	poetry run black . --exclude tests/output_
+
+clean:						  ## - Clean out generated files from the workspace
+	rm -rf .coverage \
+	       .mypy_cache \
+	       .pytest_cache \
+	       dist \
+	       **/__pycache__ \
+	       betterproto/tests/output_*
+
+# Manual testing
+
+# By default write plugin output to a directory called output
+o=output
+plugin:             ##	- Execute the protoc plugin, with output writte to `output` or the value passed to `-o`
+	mkdir -p $(o)
+	protoc --plugin=protoc-gen-custom=betterproto/plugin.py $(i) --custom_out=$(o)
+
+# CI tasks
+
+full-test: generate ##	- Run full testing sequence
+	poetry run tox
+
+check-style:        ##	- Check if code style is correct
+	poetry run black . --check --diff --exclude tests/output_