Really fix wheel build for postgis alembic, and ready for 1.9.0-rc8

[SpacemanPaul]

Reason for this pull request

Second attempt at fixing wheel build for postgis driver alembic usage (See #1597), and update whats_new for ANOTHER rc release.

Proposed changes

• Ensure alembic data files are included in wheel build

• Update whats_new.rst for 1.9.0-rc8 release

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.43%. Comparing base (6d802b8) to head (78ddd5c).
Report is 30 commits behind head on develop-1.9.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop-1.9    #1599   +/-   ##
============================================
  Coverage        85.43%   85.43%           
============================================
  Files              140      140           
  Lines            15496    15496           
============================================
  Hits             13239    13239           
  Misses            2257     2257           

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