PKGBUILD suggestions

• Replace python-keras-applications
  • Keras Applications models have moved into the core Keras repository and the TensorFlow pip package.
  • https://github.com/keras-team/keras-applications/commit/06fbeb0f16e1304f239b2296578d1c50b15a983a
• Change to SPDX license identifier
  • https://github.com/keras-team/keras/blob/v2.15.0/LICENSE
• Stop installing license file for common license Apache-2.0
• Build from source

Not included allowing backend selection.