This PR makes several changes to the `./scripts-dev/lint.sh` script, which lints the codebase with a number of tools:
* Adds usage information, with `-h` flag to show it. Otherwise it will show when providing an unknown flag.
* Adds option `-d` which will check both staged and unstaged files that have changed since the last commit and add them to the list of files to lint.
- Note that only files without an extension, or with a `.py` extension will be allowed. This prevents editing bash scripts causing the linters to break on non-python files.
* Improves the print-out of which files/directories are being linted.
Run `isort`, `flake8` and `black` over the `contrib/` directory and `synctl` script. The latter was already being done in CI, but now the linting script does it too.
Fixes https://github.com/matrix-org/synapse/issues/7910
The CI appears to use the latest version of isort, which is a problem when isort gets a major version bump. Rather than try to pin the version, I've done the necessary to make isort5 happy with synapse.
