Removes all uses of the `cgi` module from Synapse. It was deprecated in
Python version 3.11 and removed in version 3.13 ([“dead
battery”](https://docs.python.org/3.13/whatsnew/3.13.html#pep-594-remove-dead-batteries-from-the-standard-library)).

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters)

)

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.

* Clarifies comments and documentation.
* Adds type-hints.
* Fixes Python 3 compatibility (and runs pyupgrade).
* Updates for changes in Synapse internals.

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

* Fix seven contrib files with Python syntax errors

Signed-off-by: cclauss <cclauss@me.com>

Signed-off-by: Adrian Tschira <nota@notafile.com>