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

pyproject.toml

@@ -1,9 +1,58 @@
+[tool.poetry]
+name = "betterproto"
+version = "1.2.5"
+description = "A better Protobuf / gRPC generator & library"
+authors = ["Daniel G. Taylor <danielgtaylor@gmail.com>"]
+readme = "README.md"
+repository = "https://github.com/danielgtaylor/python-betterproto"
+keywords = ["protobuf", "gRPC"]
+license = "MIT"
+
+exclude = ["betterproto/tests"]
+
+[tool.poetry.dependencies]
+python = "^3.6"
+backports-datetime-fromisoformat = { version = "^1.0.0", python = "<3.7" }
+black = { version = "^19.10b0", optional = true }
+dataclasses = { version = "^0.7", python = ">=3.6, <3.7" }
+grpclib = "^0.3.1"
+jinja2 = { version = "^2.11.2", optional = true }
+protobuf = { version = "^3.12.2", optional = true }
+stringcase = "^1.2.0"
+
+[tool.poetry.dev-dependencies]
+mypy = "^0.770"
+pytest = "^5.4.2"
+pytest-asyncio = "^0.12.0"
+pytest-cov = "^2.9.0"
+tox = "^3.15.1"
+
+[tool.poetry.scripts]
+protoc-gen-python_betterproto = "betterproto.plugin:main"
+
+[tool.poetry.extras]
+compiler = ["black", "jinja2", "protobuf"]
+
 [tool.black]
 target-version = ['py36']
 
-[tool.isort]
-multi_line_output = 3
-include_trailing_comma = true
-force_grid_wrap = 0
-use_parentheses = true
-line_length = 88
+[tool.coverage.run]
+omit = ["betterproto/tests/*"]
+
+[tool.tox]
+legacy_tox_ini = """
+[tox]
+isolated_build = true
+envlist = py36, py37
+
+[testenv]
+whitelist_externals = poetry
+commands =
+    poetry install -v --extras compiler
+    poetry run pytest --cov betterproto
+"""
+
+[build-system]
+requires = ["poetry>=0.12"]
+build-backend = "poetry.masonry.api"
+