Fix python scripts to work with python3 (#2426)

Updated script to work with python3 and python2.

Added required tools.

We added a section to the readme to mention the tools that are needed to
build and test spirv-tools. For the compiler, the compilers used by the
bots are mentioned.

The bots have been changed. The windows bots will not use python 3.6 for testing. The other bots will still use python 2.7. Both Python2 and Python3 will be tested.

Fixes #2407.
Fixes #1856.