3.0 backwards compatibility breaks

[lemon24]

This is to track all the backwards compatibility breaks we want to do in 3.0.

Things that require deprecation warnings pre-3.0:

• (done in 2.8) deprecate old feed metadata / tag methods; Consider merging tags and metadata #266 (comment)
  • delete methods and corresponding documentation
• rename object_id to resource_id and make make FeedError.resource_id tuple[str]; Consider merging tags and metadata #266 (comment)
  • delete *.object_id

Things that do not require / can't have deprecation warnings pre-3.0:

• (done in 2.7) rename the mark_as_read config tag from .reader.mark_as_read to .reader.mark-as-read
  • remove pre-2.7 config migration
• delete old metadata exceptions; Consider merging tags and metadata #266 (comment)
• drop all 2.x migrations; the upgrade path from reader 2.x to reader 3 will require upgrading to reader 2.latest first
• use positional-only parameters, where possible (after Drop Python 3.7 support #278 is done)
  • in theory, it is possible to raise deprecation warnings for this (example), it's just too much work
• TagError(key, resource_id) -> TagError(resource_id, key)
• plugin_func(reader) exceptions in make_reader() should be wrapped in PluginError

[lemon24]

To do for object_id -> resource_id:

• add new, deprecate old
• update lib usage and tests
• rename storage ResourceId arguments
• update web app usage and tests
• changelog (include TagError signature change?)
• guide
• final grep, diff check

[lemon24]

3.0 released, resolving.