Use a vendored Pep562 backport to simulate Python 3.7's new PEP 562 feature. For Python3.7 and later, default to the official implementation.

This essentially implements the closest we can get to PEP 562 which allows for modules to control `__dir__` and `__getattr__` in order to deprecate attributes. Here we provide a wrapper class for the module in `util`. If a module has attributes that need to deprecated, we derive from the wrapper class and define the attributes as functions with the `property` decorator and the provided `deprecated` decorator. The class is instantiated with the module's `__name__` attribute and the class will properly replace the module with the wrapped module.  When accessing the depracted attributes, a warning is raised. Closes #739.

Update version format to be PEP 440 compliant in preparation for #732.

Add `pyproject.toml` file and configure tox to use it. Define requirement for `setuptools>=36` which validates and normalizes the version format.

Related to #709.

... for backward compatibility with older (<3.0) versions.

Fixes #435.