The `__future__` we relied on is now, where the 3 specific things are
all included [since Python 3.0](https://docs.python.org/3/library/__future__.html):
* absolute_import
* print_function
* unicode_literals
* division
These import statements are no-ops and are no longer necessary.
The library `six` provides compatibility between Python 2, and 3. It's no
longer necessary once we migrate of Python 2 completely.
Also remove any custom logic for Python 2 (the ones referenced by
a commentanyways).
https://bugs.swift.org/browse/SR-16025
Having the test directory match the module we are testing means we can have scripts in the top level of utils/build_swift which can also have tests. As part of this re-structure the test utilties have been simplified somewhat and all tests no longer use a custom TestCase, rather the standard one exposed by the unittest module.
* Implemented a presets module which includes a more robust and easy to understand parser. Moved the swift-sdks migration code to a new migration module and added testing for both the presets and migration modules. Also converted build-script to use the new presets parser.
* Switched the expansion algorithm to resolve mixins in-place (little known feature) and also changed the parser to skip all non-preset sections. Tests are included for these two behaviors.
* Re-worked the presets error hierarchy to have more descriptive and information packed exception classes. Also re-worked the PresetParser to catch duplicate preset declarations and duplicate options in a single preset. There's some special shim-code to handle the disconnect between the Python 2 ConfigParser module and the Python 3 update which adds DuplicateOptionError.
* Implemented a wrapper module around the standard argparse package, exposing the same interface with some extras on top, including a new builder type with expressive DSL for constructing complex argument parsers.
* Fixed imports in build_swift/argparse/__init__.py to make flake8 happy.
* More re-formmating to meet the exacting standards of the python_lint script.
* Added doc-strings to all the modules in the build_swift argparse overlay.
* Implemented a new BoolType for the argparse module which handles boolean-like values and replaces the hard-coded boolean values in the _ToggleAction class.
* Fixed the mess of imports in the tests sub-package to favor relative imports, so now the unit-tests will actually run as expected. The README has also been updated with a better command for executing the unit-test suite.
* Updated the add_positional method on the ArgumentParser builder class to only take a single action or default to the store action.
* Cleaned up the set_defaults method.
* Added validation test to run the build_swift unit-tests.
* Updated validation-test for the build_swift unit-test suite to use %utils.
* Fixed hard-coded default values in the expected_options module used for generating argument parser tests.
* Updated the comment in the Python validation test to run the build_swift unit-tests.
