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

README.md

@@ -46,10 +46,10 @@ First, install the package. Note that the `[compiler]` feature flag tells it to
 
 ```sh
 # Install both the library and compiler
-$ pip install "betterproto[compiler]"
+pip install "betterproto[compiler]"
 
 # Install just the library (to use the generated code output)
-$ pip install betterproto
+pip install betterproto
 ```
 
 Now, given you installed the compiler and have a proto file, e.g `example.proto`:
@@ -68,7 +68,7 @@ message Greeting {
 You can run the following:
 
 ```sh
-$ protoc -I . --python_betterproto_out=. example.proto
+protoc -I . --python_betterproto_out=. example.proto
 ```
 
 This will generate `hello.py` which looks like:
@@ -299,25 +299,26 @@ datetime.datetime(2019, 1, 1, 11, 59, 58, 800000, tzinfo=datetime.timezone.utc)
 
 Join us on [Slack](https://join.slack.com/t/betterproto/shared_invite/zt-f0n0uolx-iN8gBNrkPxtKHTLpG3o1OQ)!
 
-First, make sure you have Python 3.6+ and `pipenv` installed, along with the official [Protobuf Compiler](https://github.com/protocolbuffers/protobuf/releases) for your platform. Then:
+First, make sure you have Python 3.6+ and `poetry` installed, along with the official [Protobuf Compiler](https://github.com/protocolbuffers/protobuf/releases) for your platform. Then:
 
 ```sh
 # Get set up with the virtual env & dependencies
-$ pipenv install --dev
+poetry install
 
-# Link the local package
-$ pipenv shell
-$ pip install -e .
+# Activate the poetry environment
+poetry shell
 ```
 
+To benefit from the collection of standard development tasks ensure you have make installed and run `make help` to see available tasks.
+
 ### Code style
 
 This project enforces [black](https://github.com/psf/black) python code formatting.
 
 Before commiting changes run:
 
-```bash
-pipenv run black .
+```sh
+make format
 ```
 
 To avoid merge conflicts later, non-black formatted python code will fail in CI.
@@ -351,10 +352,16 @@ Here's how to run the tests.
 
 ```sh
 # Generate assets from sample .proto files
-$ pipenv run generate
+make generate
 
 # Run all tests
-$ pipenv run test
+make test
+```
+
+To run tests as they are run in CI (with tox) run:
+
+```sh
+make full-test
 ```
 
 ### (Re)compiling Google Well-known Types