AURORA-193: License header cleanup

Reverting the gradle license header plugin from commit b493b91. The
sourceSet exludes is modifying the global sourceSet and causing the
tests to break.
AURORA-157: Reorder python imports to be consistent

Move python imports to adhere to the following order:
- standard library deps
- apache.* deps
- gen.* (code generated) deps
- within-package (.-prefixed) deps
- third party deps

Testing Done:
./gradlew build && ./pants src/test/python:all -vxs

Bugs closed: AURORA-157

Reviewed at https://reviews.apache.org/r/17734/