chore: fix clock util import path

[itsoyou]

From synapse version v1.139.2_1, clock path from synapse.util import Clock is no longer available

element-hq/synapse#18944

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (c39a397) to head (9c6f75d).
⚠️ Report is 2 commits behind head on release/v0.4.10.

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v0.4.10     #103   +/-   ##
================================================
  Coverage            97.45%   97.45%           
================================================
  Files                    7        7           
  Lines                  669      669           
  Branches                97       97           
================================================
  Hits                   652      652           
  Misses                  17       17           

Files with missing lines	Coverage Δ
synapse_invite_checker/invite_checker.py	96.47% <100.00%> (ø)

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[jason-famedly]

LGTM! 🚀