v0.3.4

This is a backward compatible major release that adds several new features.

New features and enhancements:

• Added support for customizing the contrasts generated for categorical
  features, including treatment, sum, deviation, helmert and custom contrasts.
• Added support for the generation of linear constraints for ModelMatrix
  instances (see ModelMatrix.model_spec.get_linear_constraints).
• Added support for passing ModelMatrix, ModelSpec and other formula-like
  objects to the model_matrix sugar method so that pre-processed formulae can
  be used.
• Improved the way tokens are manipulated for the right-hand-side intercept and
  substitutions of 0 with -1 to avoid substitutions in quoted contexts.

Bugfixes and cleanups:

• Fixed variable sanitization during evaluation, allowing variables with
  special characters to be used in Python transforms; for example:
  bs(`my|feature%is^cool`).
• Fixed the parsing of dictionaries and sets within python expressions in the
  formula; for example: C(x, {"a": [1,2,3]}).
• Bumped requirement on astor to >=0.8 to fix issues with ast-generation in
  Python 3.8+ when numerical constants are present in the parsed python
  expression (e.g. "bs(x, df=10)").