This converts the `utils/api_checker` scripts to support Python 3 only.
It removes all `from __future__` imports, and switches any `python`
shebangs to `python3`.
* More Python3 lint fixes
Some of the issues addressed include:
* Don't use `l` as a variable name (confusable with `1` or `I`)
* `print` statement does not exist in Py3, use `print` function instead
* Implicit tuple deconstruction in function args is no longer supported,
use explicit splat `*` at the call site instead
* `xrange` does not exist in Py3, use `range` instead
* Better name per review feedback
Based on the same concept of swift-api-dump, this python wrapper
provides framework authors a convenient way to generate baseline or
diagnose ABI/API breakages from an existing toolchain where it has
swift-api-digester installed.
cmake installation part will be added in a separate PR.
rdar://problem/29649661
