Use repo_handler instead of pr_handler to get config in pull request branch

[astrofrog]

This is due to bug in PullRequestHandler (OpenAstronomy/baldrick#41)

[codecov]

Codecov Report

Merging #98 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage    98.6%   98.61%   +<.01%     
==========================================
  Files           3        3              
  Lines         143      144       +1     
==========================================
+ Hits          141      142       +1     
  Misses          2        2

Impacted Files	Coverage Δ
astropy_bot/tests/test_changelog_checker.py	100% <100%> (ø)	⬆️
astropy_bot/changelog_checker.py	95% <100%> (ø)	⬆️

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