[mblack] Ignore non mojo files

For context, in #3832 we decided
that mblack (aka mojo format) will no longer format non mojo files. The
change is trivial fortunately.
However, this doesn't solve the problem if formatting mojo cells in
notebooks, but that's a problem for the future.

MODULAR_ORIG_COMMIT_REV_ID: a28bc715affc0d83bff92b72c7eb313e1f80db96

docs/changelog.md

@@ -24,6 +24,9 @@ what we publish.
 
 ### Tooling changes
 
+- mblack (aka `mojo format`) no longer formats non-mojo files. This prevents
+  unexpected formatting of python files.
+
 ### ❌ Removed
 
 ### 🛠️ Fixed