Install typing stubs for boto3 and botocore.

[wRAR]

Also installs pytest and switches attrs and Pillow from separate stubs to built-in ones following the separate stub package descriptions.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.80%. Comparing base (180bc9b) to head (4b16685).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6370      +/-   ##
==========================================
+ Coverage   85.00%   89.80%   +4.80%     
==========================================
  Files         161      162       +1     
  Lines       11962    12113     +151     
  Branches     1872     1901      +29     
==========================================
+ Hits        10168    10878     +710     
+ Misses       1512      927     -585     
- Partials      282      308      +26     

Files	Coverage Δ
scrapy/core/downloader/handlers/s3.py	94.54% <100.00%> (+65.45%)	⬆️
scrapy/extensions/feedexport.py	93.13% <60.00%> (+9.38%)	⬆️

... and 20 files with indirect coverage changes