Remove the deprecated cgi module (#17741)

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>

changelog.d/17741.misc

@@ -0,0 +1 @@
+Remove usage of the deprecated cgi module.

contrib/graph/graph.py

@@ -20,8 +20,8 @@
 #
 
 import argparse
-import cgi
 import datetime
+import html
 import json
 import urllib.request
 from typing import List
@@ -85,7 +85,7 @@ def make_graph(pdus: List[dict], filename_prefix: str) -> None:
             "name": name,
             "type": pdu.get("pdu_type"),
             "state_key": pdu.get("state_key"),
-            "content": cgi.escape(json.dumps(pdu.get("content")), quote=True),
+            "content": html.escape(json.dumps(pdu.get("content")), quote=True),
             "time": t,
             "depth": pdu.get("depth"),
         }

synapse/http/matrixfederationclient.py

@@ -19,7 +19,6 @@
 #
 #
 import abc
-import cgi
 import codecs
 import logging
 import random
@@ -1813,8 +1812,9 @@ def check_content_type_is(headers: Headers, expected_content_type: str) -> None:
         )
 
     c_type = content_type_headers[0].decode("ascii")  # only the first header
-    val, options = cgi.parse_header(c_type)
-    if val != expected_content_type:
+    # Extract the 'essence' of the mimetype, removing any parameter
+    c_type_parsed = c_type.split(";", 1)[0].strip()
+    if c_type_parsed != expected_content_type:
         raise RequestSendFailed(
             RuntimeError(
                 f"Remote server sent Content-Type header of '{c_type}', not '{expected_content_type}'",