Migrate to Python 3.7+

[orome]

Migrate all code to Python 3.7+ (abandoning 2.7), including:

• Run 2to3 on all files and fix or disable remaining errors
• Add type annotations to all package functions and declarations
  • Try pytype?
• Review all annotations for semantics, especially list vs str vs Mapping etc.
  • Fix all inconsistencies in how list vs str is used
• Fix all introduced bugs:
  • Specifying step does not override number of steps implicit in message when longer (possibly old bug/feature: check docs and sync with Python implementation)
  • Some tests fail: reenable and fix all disabled tests
  • Bugs in docs
• Update handling of Unicode (change caveat about Windows, etc.); confirm ability to run on Windows (see and close Confirm correct command line encoding #6, Command line display of debug and config formats reports Unicode error #30, Provide better handling of preference for Unicode #1)
• Update docs with correct handling of types; ensure synchronization with annotations
• Add new tests (see also Add tests for exceptions to testing scripts #11 , Add tests for printing functions to testing scripts #10)
• Expand to Python 3.8 (and test on Travis)
• Look for opportunities to use itertools, (and other package?) for mappings and encodings

Also general bug review and syncing with Haskell version:

• Check formatting of CLI encode output (see Haskell version #33).