Improve config handling

[fcollonval]

Improve unit tests

This has been extracted from #44

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@02e2cbd). Click here to learn what that means.
The diff coverage is 93.27%.

@@            Coverage Diff            @@
##             master      #47   +/-   ##
=========================================
  Coverage          ?   60.86%           
=========================================
  Files             ?       34           
  Lines             ?     1661           
  Branches          ?       94           
=========================================
  Hits              ?     1011           
  Misses            ?      645           
  Partials          ?        5           

Impacted Files	Coverage Δ
jupyterlab_pullrequests/handlers.py	86.60% <64.28%> (ø)
...ab_pullrequests/tests/test_handlers_integration.py	90.62% <83.33%> (ø)
jupyterlab_pullrequests/managers/manager.py	89.21% <85.71%> (ø)
jupyterlab_pullrequests/tests/conftest.py	96.00% <95.83%> (ø)
jupyterlab_pullrequests/managers/github.py	84.61% <100.00%> (ø)
jupyterlab_pullrequests/managers/gitlab.py	80.64% <100.00%> (ø)
jupyterlab_pullrequests/tests/test_github.py	100.00% <100.00%> (ø)
jupyterlab_pullrequests/tests/test_gitlab.py	100.00% <100.00%> (ø)
jupyterlab_pullrequests/tests/test_manager.py	98.36% <100.00%> (ø)

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[bollwyvl]

LGTM