Python 3+ release

Major change: drop compatibility with Python 2, which allows significant
code cleanups.

Other changes:

• Switch internal implementation of argument parsing to a built-in one
  (PyUnicode_FSConverter), which brings automatic support for
  path-like objects in Python 3.6+ (#20), and also a more uniform
  handling of Unicode path arguments with respect to other Python code.
• Fix missing error check in list operations in get_all (#17).
• Switch test library to pytest; not that a reasonable recent version is
  needed. Additionally, expand test coverage, although not directly
  visible in actual coverage reports…

Version 0.7.1 is just 0.7.0 (the above changes) plus a minor typo fix.