Switch to Poetry & use Github Actions

[mtraynham]

I believe this project is pretty much inactive (#1719) and I'm not sure who owns it (best guess @kragniz / @stephenfin).

This PR brings some of the tooling up to date.

I initially wanted to fork and update the proto/grpc versions (#2045), but the support for Python 2.7, 3.5, 3.6, etc. make it hard to do so. On top of this, some of the testing with pifpaf/subprocess seems a bit flakey. Therefore, I simply pinned all the versions to what was currently in requirements/test.txt and got as many green tests as I could; Python 3.7-3.9 with latest etcd 3.0.X, 3.1.X, 3.2.X, 3.3.X, 3.4.X, 3.5.X.

I likely plan to fork and drop support for Python/etcd/grpc/protobuf versions to accommodate my own app, although I would prefer not to. It's probably inappropriate to have a public library target one specific Python version/etcd version...

This uses Poetry, which is based on PEP 621 – Storing project metadata in pyproject.toml. It also uses Github Actions instead of Travis CI to perform the builds. Tox is dropped simply in favor of utilizing a Makefile and using Github Action matrix testing. If you need to utilize different versions of Python, I'd recommend pyenv or asdf.

With the current setup, testing <3.7 isn't supported by Github Actions & 3.10/3.11 are avoided because the grpc versions don't have wheels.

$ pip install poetry==1.2.2
$ make install lint test clean uninstall
$ make help
──────────────────────── Make Commands ────────────────────────────────
install: install local development dependencies
uninstall: uninstalls local development dependencies
clean: removes intermediate files and build artifacts
lint: lints all code
test: runs the tests
build: builds the project to tar & wheel formats
.etcd: installs etcd locally for running
etcd3/proto: clones the protobuf definitions
etcd3/etcdrpc: generates etcd protobuf definitions
etcd-up: starts etcd
etcd-down: stops etcd
help: show this help

Override etcd version

$ TEST_ETCD_VERSION=v3.5.6 make clean test

Regenerate protobuf

$ make etcd3/etcdrpc

Run etcd in a container

$ make etcd-up  # make etcd-down

Some of the tests seem to fail intermittently, but I wanted to avoid test alterations for tooling only.