Ignore reformatting revision (#794)

Follow-on work to #774, this PR uses a [`.git-blame-ignore-revs`
file](https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view)
to ignore revs that cause major reformatting changes in git blame view.

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# Reformat with black, isort, docformatter, etc.
+e40ac28317c61ea90345d3499986957b0e1c9134