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:

• [x] (done in 2.8) deprecate old feed metadata / tag methods; https://github.com/lemon24/reader/issues/266#issuecomment-1013740696
  • [x] delete methods and corresponding documentation
• [x] rename object_id to resource_id and make make FeedError.resource_id tuple[str]; https://github.com/lemon24/reader/issues/266#issuecomment-1013740696
  • [x] delete *.object_id

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

• [x] (done in 2.7) rename the mark_as_read config tag from .reader.mark_as_read to .reader.mark-as-read
  • [x] remove pre-2.7 config migration
• [x] delete old metadata exceptions; https://github.com/lemon24/reader/issues/266#issuecomment-1013740696
• [x] drop all 2.x migrations; the upgrade path from reader 2.x to reader 3 will require upgrading to reader 2.latest first
• [x] use positional-only parameters, where possible (after #278 is done)
  • in theory, it is possible to raise deprecation warnings for this (example), it's just too much work
• [x] TagError(key, resource_id) -> TagError(resource_id, key)
• [x] plugin_func(reader) exceptions in make_reader() should be wrapped in PluginError

[lemon24]

To do for object_id -> resource_id:

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

[lemon24]

3.0 released, resolving.