Properly handle branch, default to main

[jtpio]

It looks like master was hardcoded.

Nowadays, GitHub repos default to main.

[bsipocz]

Thank you!

[bsipocz]

Some of the test failures seem related (main in the heading in the diff? I don't see why that happens from the PR diff, but something is not right).
Though I fail to see how the diff for one vs 2 contributors can be related at all.

[choldgraf]

Tests were happy on the latest pass, and this PR looks good to me so I'll merge so that we don't block this on test flakiness

[jtpio]

Some of the test failures seem related (main in the heading in the diff? I don't see why that happens from the PR diff, but something is not right).

Yeah not sure either. Could it be because that some of the test data was generated before the default branch was renamed to main?
Some time ago I remember seeing some missing data when making releases with the Jupyter Releaser (which uses github-activity) for repos that switched from master to main.