Demote PT-AT from runtime dependency to dev. dependency

CHANGELOG.md

@@ -8,6 +8,8 @@ recommendations of [keepachangelog.com](http://keepachangelog.com/).
 ### Breaking Changes
 
 - Remove all methods deprecated in PT 9
+- `paper_trail-association_tracking` is no longer a runtime dependency. If you
+  use it, you must now add it to your own `Gemfile`.
 - [#1108](https://github.com/paper-trail-gem/paper_trail/pull/1108) -
   In `versions.item_type`, we now store the subclass name instead of
   the base_class.

README.md

@@ -844,10 +844,11 @@ these issues, PT-AT was extracted (see
 https://github.com/paper-trail-gem/paper_trail/issues/1070).
 
 Even though this has always been an experimental feature, we don't want the
-extraction of PT-AT to be a breaking change. So, `paper_trail` will have a
-runtime dependency on this gem and will keep running the existing tests related
-to association tracking. This arrangement will be maintained for a few years, if
-practical.
+extraction of PT-AT to be a breaking change. In PT 9, PT-AT was kept as a
+runtime dependency. In PT 10, it is a development dependency, so if you use it
+you must add it to your own `Gemfile`. We will keep PT-AT as a development
+dependency and continue running the existing tests related to association
+tracking for as long as is practical.
 
 ### 4.c. Storing Metadata
 

paper_trail.gemspec

@@ -29,15 +29,6 @@ has been destroyed.
 
   # Rails does not follow semver, makes breaking changes in minor versions.
   s.add_dependency "activerecord", [">= 4.2", "< 5.3"]
-
-  # This `PT_ASSOCIATION_TRACKING` variable is convenient for the test suite of
-  # `paper_trail-association_tracking`. Normal users of paper_trail should not
-  # set this variable. This variable may be removed in the future without
-  # warning.
-  unless ENV["PT_ASSOCIATION_TRACKING"] == "false"
-    s.add_dependency "paper_trail-association_tracking", "< 2"
-  end
-
   s.add_dependency "request_store", "~> 1.1"
 
   s.add_development_dependency "appraisal", "~> 2.2"
@@ -45,6 +36,7 @@ has been destroyed.
   s.add_development_dependency "ffaker", "~> 2.8"
   s.add_development_dependency "generator_spec", "~> 0.9.4"
   s.add_development_dependency "mysql2", "~> 0.4.10"
+  s.add_development_dependency "paper_trail-association_tracking", "< 2"
   s.add_development_dependency "pg", "~> 0.21.0"
   s.add_development_dependency "rake", "~> 12.3"
   s.add_development_dependency "rspec-rails", "~> 3.7.2"