Remove confusing `aiobotocore.session.Session` symbol

[jakob-keller]

Description of Change

Rename import of botocore.session.Session in aiobotocore.session to mark it private.

Assumptions

aiobotocore.session.Session was never intended to become part of public API.

Checklist for All Submissions

• [x] I have added change info to CHANGES.rst
• [x] If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details): closes #972

[codecov[bot]]

Codecov Report

Merging #1047 (075c980) into master (71bb001) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1047   +/-   ##
=======================================
  Coverage   86.39%   86.39%           
=======================================
  Files          60       60           
  Lines        5805     5807    +2     
=======================================
+ Hits         5015     5017    +2     
  Misses        790      790           

Flag	Coverage Δ
unittests	86.39% <100.00%> (+<0.01%)	:arrow_up:

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Files	Coverage Δ
aiobotocore/session.py	79.24% <100.00%> (+0.39%)	:arrow_up:

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[thehesiod]

hrm, github resolve isn't so great

[thehesiod]

feel free to re-merge and force push up

[jakob-keller]

feel free to re-merge and force push up

CI is running 🤞🏻