Shed

- Fix latent bugs around min-version handling with Black and LibCST
- Fix a rare crash with `--refactor` where com2ann could fail to parse input

- Continue in a single process if `os.fork()` fails with `BlockingIOError`

- Stop removing "unused" imports in code blocks; often they're used in later blocks
- Use custom LibCST refactoring passes instead of `pybetter`; because these are faster
they're also enabled outside of `--refactor` mode
- Replace `raise NotImplemented` with `raise NotImplementedError`
- Replace `assert False [, msg]` with `raise AssertionError [(msg)]`, for all falsey
literals, to avoid surprising behaviour with `PYTHONOPTIMIZE`
(use `if __debug__` if you need such behaviour)
- Remove `assert True`, or other truthy literals, which can never fail

- Warn on invalid syntax, and try to format docstrings anyway
([23](https://github.com/Zac-HD/shed/issues/23))

- Skip files or code-blocks with invalid syntax
([23](https://github.com/Zac-HD/shed/issues/23))

- Add new `--py-37-plus` (etc) arguments to select the minimum Python version
- disable `autoflake`'s remove-unused-variables; allow linters to warn about probable bugs