[libFuzzer] Make DataFlow scripts Python3 compatible.

Summary:
Python2 will hit end of life soon: https://pythonclock.org/
This change also makes the integration with OSS-Fuzz a bit simpler:
https://github.com/google/oss-fuzz/issues/1632

Reviewers: morehouse, kcc

Reviewed By: morehouse

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60282

llvm-svn: 357726