API changes:
* Drop Python 3.4 and 3.5; add Python 3.7 and 3.8
* `lifetime` setting can take an integer or a timedelta
* Don't expose `public` in the package namespace
* Use exception chaining where appropriate
* Add type annotations and enforce with mypy

Testing:
* Switch to pytest as the testing framework
* Get coverage working through pytest-cov plugin
* diffcov appears to be no longer supported, so sadly ditching it.
* Adjust for new timedelta reprs

Other changes:
* Move flake8 settings to setup.cfg
* Fix flake8 invocation
* Adopt isort instead of flufl.testing flake8 warnings
* Fix namespace package declaration
* Boost coverage
* Bump copyright years
* Bump version
* IOError -> OSError