Release 0.5.4

Maintenance release:

• Switch build system to Python 3 by default (can be overridden if
  needed).
• Internal improvements for better cpychecker support.
• Fix compatibility with PyPy.
• Test improvements (both local and on Travis), testing more variations
  (debug, PyPy).
• Improve test coverage, and allow gathering test coverage results.
• Drop support (well, drop testing) for Python lower than 2.7.
• Minor documentation improvements (closes #9, #12).