Add @final/Final type hints

[asvetlov]

No description provided.

[codecov]

Codecov Report

Merging #477 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #477   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files           2        2           
  Lines         689      689           
  Branches      154      154           
=======================================
  Hits          684      684           
  Misses          5        5           

Impacted Files	Coverage Δ
yarl/__init__.py	99.04% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b875e4...72f6848. Read the comment docs.