Fix type error in titer model cross-validation

[huddlej]

Description of proposed changes

Fixes type errors in titer model code that is only accessible through the developer's Python API but which prevented me from running cross-validation of titer models with that API. It looks like these were the only references to the dictionary methods of keys and values that didn't get updated in the past.

Related issue(s)

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 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.30%. Comparing base (e90383b) to head (3a6df05).

Files with missing lines	Patch %	Lines
augur/titer_model.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1688   +/-   ##
=======================================
  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.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests