This is the last version to support Python 2.6.

Changes since 3.0:

* Support for sigtools' automatic signature discovery. This is reflected
  in the function composition tutorial: In most cases you no longer have
  to specify how your decorators use `*args` and `**kwargs` exactly
* You can supply 'alternative actions' (i.e. --version) even when using
  multiple commands.
* Improve hackability of argument parsing: named parameters are now
  sourced from the bound arguments instance, so a parameter could modify
  it duing parsing without changing the original signature.