Use jsonschema ≥4.18.0 and new referencing library

[victorlin]

Description of proposed changes

In v4.18.0, jsonschema.RefResolver was deprecated in favor of the new referencing library. The intro and API docs were helpful in determining the necessary changes.

I've tested that our new usage is not backwards compatible with v4.17.3 and thus updated the minimum requirement to v4.18.0. I chose v0.29.1 as the minimum supported version of referencing because that was the version released alongside jsonschema v4.18.0.

The default behavior no longer tries to access the network, so I've reworded the retrieval function comment and error message.

Local reference mismatches are now a "PointerToNowhere" error instead of an "Unresolvable JSON pointer" error. It shows the entire schema JSON in the output which can seem unnecessarily verbose, but I think it's fine since this is only intended to show on internal errors with the schema.

Related issue(s)

Closes #1358

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.30%. Comparing base (e90383b) to head (901fcc8).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/validate.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1691   +/-   ##
=======================================
  Coverage   72.30%   72.30%           
=======================================
  Files          79       79           
  Lines        8276     8276           
  Branches     1691     1691           
=======================================
  Hits         5984     5984           
  Misses       2007     2007           
  Partials      285      285           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[huddlej]

Thanks, @victorlin! I love an improvement that's +5 lines and -14 😄

Just adding a note here for our own reference that we'll need to add referencing to the Bioconda package dependencies and update the jsonschema version pin there.

addressed requested changes