Use requirements.txt for 3rdparty/python.

Also drop an unused Flask dependency and upgrade CherryPy to 3.6.0
(which has a functioning version on PyPI and thus doesn't need a
custom repository set in the BUILD file).

Testing Done:
./pants src/test/python:all

Bugs closed: AURORA-617

Reviewed at https://reviews.apache.org/r/28610/