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. 

moves non-runtime dependencies out of synapse.python_dependencies (test and lint)

Synapse supports Python 3.8. We've been using it in CI for a while now.

* remove 2.7 from CI and publishing

* fill out classifiers and also make it not be installed on 3.5

* some minor bumps so that the old deps work on python 3.5

Using systemd-python allows for logging to the systemd journal,
as is documented in: `synapse/contrib/systemd/log_config.yaml`.

Signed-off-by: Silke Hofstra <silke@slxh.eu>

Make the dependencies more like a standard Python project and hook up the optional dependencies to setuptools (#4298)

Using tox to run the tests is a bad idea, as per the comments.

Seperate scripts/ into scripts/ and scripts-dev/, where scripts/* are automatically added to the package

Single source version and python dependencies, prevent people accidentally installing with easy_install, use scripts rather than entry_points to install synctl