This completely removes all objects which were deprecated in version 3.0 (this change will be included in version 3.4). Given the time that has passed, and the fact that older unmaintained extensions are not likely to support the new minimum Python version, this is little concern about breaking older extensions.

Python dropped support on 2021-12-23. Our policy (#760) is to drop
support on the next point release after Python does.

* Remove py36 tests
* Test multiple recent versions of pypy
* Remove pep562 backport

Ensures anyone still using Python 2 gets a readable error message.
This does not specify a specific version number to remove the maintenance
burden of updating it with each future release. It will likely get removed
in the next few releases as it will have served its purpose by then.
Fixes #871.

* Python syntax upgraded using `pyupgrade --py3-plus`
* Travis no longer uses `sudo`. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

See #760 for Python Version Support Timeline and related dicussion.

 Add pep517check environment to tox

 Split version info into a separate file, load it using importlib

Fixes #823.

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.