Explicitly set PR trigger for all branches on Azure

[leouieda]

For some reason, Azure Pipelines CI wasn't starting up PRs, only on master and tags. Explicitly setting a trigger for PRs against any branch seems to work.

Reminders:

• [ ] Run make format and make check to make sure the code follows the style guide.
• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• [ ] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• [ ] Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[codecov[bot]]

Codecov Report

Merging #28 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #28   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines         102    102           
  Branches        1      1           
=====================================
  Hits          102    102

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