Reformat all files with the new version.

Signed-off-by: Marcus Hoffmann <bubu@bubu1.eu>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Drop Python 3.5 from Trove classifier metadata.

Signed-off-by: Dan Callahan <danc@element.io>

This fixes an error ("Cannot determine consistent method resolution order (MRO)")
when running mypy with a cache.

* Adds B00 to ignored checks.
* Fixes remaining issues.

 - Update black version to the latest
 - Run black auto formatting over the codebase
    - Run autoformatting according to [`docs/code_style.md
`](https://github.com/matrix-org/synapse/blob/80d6dc9783aa80886a133756028984dbf8920168/docs/code_style.md)
 - Update `code_style.md` docs around installing black to use the correct version

This new version no longer has the problem of adding/removing a blank line in `.pyi` files, which black disagrees with. This would cause `isort` to slightly modify `.pyi` files, before `black` would subsequently modify back directly afterwards.

Relevant `isort` issue: https://github.com/pycqa/isort/issues/1284

This avoids a warning when uploading packages to PyPI via twine.

As expected, all tests pass locally without modification.

Signed-off-by: Dan Callahan <danc@element.io>

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