gitattributes: allow diff of test data (#470)

* gitattributes: allow diff of data.py files

* Allow diffs of all text files, not just data.py

.gitattributes

@@ -1,2 +1,4 @@
 # Do not change line endings on test data, it will change the MD5
-/tests/data/*/** binary
+/tests/data/*/** -text
+# Test data generation files are fine though
+/tests/data/**/data.py text