Tox-poetry-installer

- Add `poetry` extra to support installing Poetry as a direct dependency of the plugin
- Add `--require-poetry` runtime option to force Tox failure if Poetry is not installed
- Update logging messages to improve UX around non-verbose messaging
- Update error logging to avoid dumping stack traces
- Update integration with Tox's `action` object to better manage internal state at runtime
- Update documentation to more clearly cover more use cases
- Remove `poetry` as a required dependency to support external Poetry installations

First beta release :tada:

- Fix always attempting to install dependencies with incompatible python version
constraints
- Fix always attempting to install dependencies with incompatible python platforms

- Add CI/Tox tests for Python-3.9
- Update dependency processing to reduce duplication during installation
- Update minimum python requirement to `3.6.1`
- Fix `UnboundLocal` exception when not installing project dependencies

- Add option `locked_deps` to better support both locked and unlocked dependencies in a
single environment
- Add blocking functionality when using `require_locked_deps = true` to prevent other
hooks from running after this one
- Update documentation to include new configuration options and errors
- Update documentation to improve future maintainability
- Update module structure to move from single-file module to multi-file directory module
- Fix `RecursionError` when installing locked dependencies that specify recursive
dependencies
- Fix always reinstalling all locked dependencies on every run regardless of update status

- Add `install_dev_deps` configuration option for automatically installing all Poetry
dev-dependencies into a Tox testenv

- Fix error when installing an environment with no extras specified in the configuration
- Fix problem where only the dependencies of the sequentially last extra would be
installed
- Fix regression causing no project dependencies to be installed