Update deprecated_at_birth script

Make it executable, make it work with python3.

Bug: 228451704
Test: $ m api_txt dist
      $ PREVIOUS_API=prebuilts/sdk/##
      $ for api in $(ls -1 out/dist/apistubs/android); do
          frameworks/base/tools/apilint/deprecated_at_birth.py \
          out/dist/apistubs/android/$api/api/ \
          ${PREVIOUS_API}/$api/api/
        done
Change-Id: Ib0eda541be68b0b21ae0dea3f51a82b44f7f9e7c