tools: Revert to Python 2 typing syntax for now.

This reverts commit 66261f1cc.  See parent commit for reason; here,
provision worked but `tools/run-dev.py` would give errors.

We need to figure out a test that reproduces these issues, then make a
version of these changes that keeps that test working, before we
re-merge them.

tools/test-tools

@@ -17,7 +17,8 @@ if __name__ == '__main__':
                         default=False, help='compute test coverage')
     args = parser.parse_args()
 
-    def dir_join(dir1: str, dir2: str) -> str:
+    def dir_join(dir1, dir2):
+        # type: (str, str) -> str
         return os.path.abspath(os.path.join(dir1, dir2))
 
     tools_dir = os.path.dirname(os.path.abspath(__file__))