Sliding Sync: Only send rooms with updates down sliding sync

changelog.d/17447.feature

@@ -0,0 +1 @@
+Track which rooms have been sent to clients in the experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.

changelog.d/17479.misc

@@ -0,0 +1 @@
+Do not send down empty room entries down experimental sliding sync endpoint.

pyproject.toml

@@ -201,8 +201,8 @@ netaddr = ">=0.7.18"
 # add a lower bound to the Jinja2 dependency.
 Jinja2 = ">=3.0"
 bleach = ">=1.4.3"
-# We use `Self`, which were added in `typing-extensions` 4.0.
-typing-extensions = ">=4.0"
+# We use `assert_never`, which were added in `typing-extensions` 4.1.
+typing-extensions = ">=4.1"
 # We enforce that we have a `cryptography` version that bundles an `openssl`
 # with the latest security patches.
 cryptography = ">=3.4.7"