Fix type annotation for TOMLEncoder.encode return value

[EliasStar]

fixes #328

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.92%. Comparing base (f1b9e07) to head (6617b6b).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #329   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         101      101           
  Lines       12007    12007           
=======================================
  Hits        11998    11998           
  Misses          9        9           

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[Fatal1ty]

Thanks!