This introduces four functions for helping deprecate function-related

code:


@deprecate_non_keyword_only_args takes a function with keyword-only

  arguments and allows them to be called with positional arguments while

  emitting a deprecation warning. This is a port of the code used in

  RBTools and Djblets today.


@func_deprecated takes a function and emits a deprecation warning

  when called.


@func_moved takes a function and emits a deprecation warning

  when called, pointing to a new function elsewhere.


deprecated_arg_value wraps a value and emits a deprecation warning

  if it's used in any way. It's useful for passing old arguments to

  callbacks. This is a port of the code used in Djblets today.


Unit tests have been added for all of these functions.

All unit tests pass for all supported versions of Python.
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