Modernize the library

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