Update return type of Resource.__iter__()

[Dreamsorcerer]

_routes doesn't declare the type as AbstractRoute, so we should match it in __iter__().

[codecov]

Codecov Report

Merging #6460 (55557a5) into master (eda8114) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6460   +/-   ##
=======================================
  Coverage   93.35%   93.35%           
=======================================
  Files         104      104           
  Lines       30484    30484           
  Branches     3067     3067           
=======================================
  Hits        28457    28457           
  Misses       1850     1850           
  Partials      177      177           

Flag	Coverage Δ
unit	93.27% <100.00%> (ø)

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Impacted Files	Coverage Δ
aiohttp/web_urldispatcher.py	97.54% <100.00%> (ø)

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[patchback]

Backport to 3.9: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.9/24af37c300006d9d7e05d0054ec820ee479d9f44/pr-6460

Backported as #6461

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