Flake8-annotations

Added
* (Internal) 71 Add `pep8-naming` to linting toolchain
* (Internal) Expand pre-commit hooks
* Add `black`
* Add `check-merge-conflict`
* Add `check-toml`
* Add `check-yaml`
* Add `end-of-file-fixer`
* Add `mixed-line-ending`
* Add `python-check-blanket-noqa`

Changed
* (Internal) Add argument names to `Argument` and `Function` `__repr__` methods to make the string more helpful to read

Fixed
* 70 Fix incorrect column index for missing return annotations when other annotations are present on the same line of source
* 69 Fix misclassification of `None` returning functions when they contained nested functions with non-`None` returns (thanks isidentical!)
* 67 Fix methods of nested classes being improperly classified as "regular" functions (thanks isidentical!)

Changed
* 64 Change prefix from `TYP` to `ANN` in order to deconflict with `flake8-typing-imports`

Added
* (Internal) Add test case for checking whether flake8 invokes our plugin
* 41 Add `--suppress-none-returning` configuration option to suppress `TYP200` level errors for functions that either lack a `return` statement or only explicitly return `None`.
* (Internal) Add `black` as an explicit developer requirement (codebase already adheres to `black` formatting)

Changed
* (Internal) 61 Migrate from Pipenv to Poetry for developer environment setup

Fixed
* (Internal) Add missing classifier test cases for POSONLYARGS
* Re-add the `tree` argument to the checker so flake8 identifies the plugin as needing to run

Changed
* Request source from `flake8` as lines of code rather than parsing it from the requested filename ourselves, allowing for proper parsing of `stdin` inputs
* (Internal) Remove `flake8-string-format` from dev dependencies, as `str.format()` isn't used anywhere

Fixed
* 52 Fix error when invoking with `stdin` source code instead of a filename

Added
* (Internal) Add [`pipenv-setup`](https://github.com/Madoshakalaka/pipenv-setup) as a dev dependency & CI check to ensure synchronization between `Pipfile` and `setup.py`
* (Internal) Add [tox](https://github.com/tox-dev/tox) configuration for local testing across Python versions
* (Internal) Add test for checking a single yield of TYP301 per function
* (Internal) Add coverage reporting to test suite
* (Internal) Add testing for positional only arguments

Changed
* (Internal) [`typed_ast`](https://github.com/python/typed_ast) is now required only for Python versions `< 3.8`
* (Internal) Update flake8 minimum version to `3.7.9` for Python 3.8 compatibility
* (Internal) 50 Completely refactor test suite for maintainability

Fixed
* (Internal) Fix mixed type hint tests not being run due to misnamed test class
* Fix `TYP301` classification issue where error is not yielded if the first argument is type annotated and the remaining arguments have type comments