chore: update typing of Event.signatures to work with recent synapse changes

[FrenchGithubUser]

commit in question:
element-hq/synapse@3e6bf10

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.61%. Comparing base (4da25e5) to head (3eab011).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage   95.61%   95.61%           
=======================================
  Files           7        7           
  Lines         798      798           
  Branches      118      118           
=======================================
  Hits          763      763           
  Misses         22       22           
  Partials       13       13           

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[jason-famedly]

Included a temporary commit that had the Synapse dependency point at the appropriate, pending, release branch to handle the new rust class for Signatures on Events. This is removed prior to merging.

Tests passing with the temporary commit: https://github.com/famedly/synapse-invite-checker/actions/runs/26519953413