Cleanup leftover boto2 code in S3DownloaderHandler

[laggardkernel]

Remove leftover attribute S3DownloaderHandler.conn. Seems we forgot to remove it in 5e99758.

[codecov]

Codecov Report

Merging #5209 (1efed43) into master (b22a004) will decrease coverage by 4.23%.
The diff coverage is n/a.

❗ Current head 1efed43 differs from pull request most recent head 7b02d1e. Consider uploading reports for the commit 7b02d1e to get more accurate results

@@            Coverage Diff             @@
##           master    #5209      +/-   ##
==========================================
- Coverage   88.38%   84.14%   -4.24%     
==========================================
  Files         162      162              
  Lines       10526    10523       -3     
  Branches     1522     1522              
==========================================
- Hits         9303     8855     -448     
- Misses        947     1406     +459     
+ Partials      276      262      -14     

Impacted Files	Coverage Δ
scrapy/core/downloader/handlers/s3.py	93.33% <ø> (+3.75%)	⬆️
scrapy/core/http2/stream.py	27.01% <0.00%> (-64.37%)	⬇️
scrapy/pipelines/images.py	28.07% <0.00%> (-62.29%)	⬇️
scrapy/core/http2/agent.py	36.14% <0.00%> (-60.25%)	⬇️
scrapy/core/downloader/handlers/http2.py	43.42% <0.00%> (-56.58%)	⬇️
scrapy/core/http2/protocol.py	34.17% <0.00%> (-49.25%)	⬇️
scrapy/robotstxt.py	75.30% <0.00%> (-22.23%)	⬇️
scrapy/utils/ssl.py	53.65% <0.00%> (-17.08%)	⬇️
scrapy/utils/asyncgen.py	83.33% <0.00%> (-16.67%)	⬇️
scrapy/core/downloader/contextfactory.py	75.92% <0.00%> (-11.12%)	⬇️
... and 7 more

[wRAR]

Please fix the flake8 failure.

[laggardkernel]

Fixed the linting error. There's another requests connection error raised during generating document. Seems unrelated with my code.

BTW, I have problem running tox on my local machine. pip stucks at dependencies resolving.

[Gallaecio]

Thanks!