Make the method APIs more resistant to breaking changes

[jwodder]

Idea: Require method implementations to be subclasses of base classes defined by versioningit, one base class per step. Instances of these classes will have step arguments provided as instance attributes (and thus additions to the arguments should not require updating any code), and they are executed via a run() or __call__() method.

• Other possible instance attributes of method classes:

  • the path to the project root
  • the complete versioningit configuration? (before or after processing by Config.parse_*?)
  • a Versioningit instance?

• Possible ways to supply the user parameters:

  • As a dict passed to the run() method
  • As a dict attribute on instances
    • A method class could then validate user parameters at construction (which happens while parsing the configuration, before execution) by overloading __init__.

• Cf. Hatch's plugin classes

• Problem: This won't address the need to make breaking changes to do_STEP() methods whenever a new argument is added.