Ignore formatting commits with git blame

When making commits that are strictly formatting/style changes (e.g., after running a new version of black or running pyupgrade after dropping an old Python version), add the commit hash to .git-blame-ignore-revs, so git blame can ignore the change.

For more details, see:

• https://git-scm.com/docs/git-config#Documentation/git-config.txt-blameignoreRevsFile

• pydata/pydata-sphinx-theme#713