Add a .git-blame-ignore-revs file to ignore style change commits

See https://black.readthedocs.io/en/stable/guides/introducing_black_to_your_project.html#avoiding-ruining-git-blame
and https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view

Tested that this works out of the box in the GitHub UI, and locally
after running:

    git config blame.ignoreRevsFile .git-blame-ignore-revs

Author: rgommers

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# Migrate code style to Black
+162034b12711dad54589c5dc9e75942695a7957f