tools: Enhance output of run-mypy.

This removes the excessively verbose lists of files
to be tested, and flushes the output after every print
to update the user on the current status in real time.

tools/run-mypy

@@ -105,9 +105,8 @@ if args.quick:
 # run mypy
 status = 0
 for repo, python_files in repo_python_files.items():
-    print("Running mypy for `{}`.".format(repo))
+    print("Running mypy for `{}`.".format(repo), flush=True)
     if python_files:
-        print(python_files)
         result = subprocess.call([mypy_command] + extra_args + python_files)
         if result != 0:
             status = result