Remove modattrs.py and related

Now that it has served its purpose. (Of course, it can also be
brought back from the history at any time if needed.)

Changes:

- Delete the modattrs.py script that was used to check for
  unintended changes to what module attributes existed and what
  objects they referred to, while doing the import refactoring.

- Delete the ab.diff file showing the overall diff, which was
  temporarily introduced in the previous commit.

- Remove the "a" and "b" temporary entries in .gitignore that were
  used to facilitate efficient use of modattrs.py while carrying
  out the import refactoring.

Author: EliahKagan

Diff for: .gitignore

@@ -47,7 +47,3 @@ output.txt
 
 # Finder metadata
 .DS_Store
-
-# Output files for modattrs.py (these entries will be removed soon)
-a
-b