fix: thread typing

[P403n1x87]

We fix a tying error with the thread field of AustinSample objects.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.71%. Comparing base (081980e) to head (d15f545).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
austin/stats.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #41   +/-   ##
=======================================
  Coverage   71.71%   71.71%           
=======================================
  Files          20       20           
  Lines        1821     1821           
  Branches      225      225           
=======================================
  Hits         1306     1306           
  Misses        431      431           
  Partials       84       84           

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