Forks-sync

- Corrects the `git fetch` command to a depth of 1 since we are also shallow cloning repos locally. This should greatly speed up the syncing process for larger repositories since we don't require the entire project tree

- Adds `mypy` type checking

- Use `woodchips` for logging (same behavior as before)
- Added Python type hinting and stronger test assertions

- Refactors the git operations behind the tool to no longer spawn a shell process when using the subprocess module. Removed commands to change directories and instead direct all git commands to the correct path at invocation

- Refactors the entire app to use CLI arguments instead of environment variables
- Added a `--force` flag to ensure users don't accidentally blow away their default branches unless they explicitly wanted to. Without passing this flag, the tool will run in "dry mode"
- Now restricts the tool to run with 10 threads by default
- Added additional logging

- Drops support for Python 3.6
- Swaps `mock` library for builtin `unittest.mock` library
- Formats project with `Black`