🎨🔧 Add initial Git blame suppression list

This patch adds a config recognized by GitHub that allows hiding
automated changes from the blame output to reduce the noise.

Author: webknjaz

.git-blame-ignore-revs

@@ -0,0 +1,4 @@
+# START initial autopep8-driven formatting
+commit 6b875871545042ee8414ba05f7743eb9a491f631  # Make nested funcs sparse @ `cheroot.ssl.pyopenssl`
+commit b234de9e9d3ca6a3ba5ad4efe87f2347045c19b4  # Dedent nested tuple items @ `test_conn`
+# END initial autopep8-driven formatting

.gitattributes

@@ -4,4 +4,7 @@
 # Needed for setuptools-scm-git-archive
 .git_archival.txt  export-subst
 
+# Blame ignore list entries are expected to always be appended, never edited
+.git-blame-ignore-revs  merge=union
+
 CHANGES.rst        merge=union