Update flask-session library

[kgopal492]

SUMMARY

Update flask-session to 0.8.0 to be able to use server side sessions.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (pinterest-release-4.0.2@56bf5b5). Learn more about missing BASE report.

Additional details and impacted files

@@                    Coverage Diff                     @@
##             pinterest-release-4.0.2      #33   +/-   ##
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  Coverage                           ?   78.13%           
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  Files                              ?      513           
  Lines                              ?    36135           
  Branches                           ?        0           
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  Hits                               ?    28234           
  Misses                             ?     7901           
  Partials                           ?        0           

Flag	Coverage Δ
mysql	77.89% <ø> (?)
postgres	77.99% <ø> (?)
python	78.13% <ø> (?)
sqlite	77.44% <ø> (?)

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