fix: support Python 3.11 by replacing PEP 695 type aliases with TypeAlias

[zhiyichin]

Motivation

matthewplotlib currently uses Python 3.12+ type X = Y syntax (PEP 695 type aliases) in three files: colormaps.py, colors.py, and data.py. This causes a SyntaxError on Python 3.11 and earlier, making the package unusable in those environments:

File ".../matthewplotlib/colormaps.py", line 25
    type ContinuousColorMap = Callable[
         ^^^^^^^^^^^^^^^^^^
SyntaxError: invalid syntax

The project currently declares requires-python = ">=3.10" and lists Python 3.10 and 3.11 in its classifiers, so the package should remain importable on those versions. There is no functional reason to require Python 3.12 for aliases that are only used as type annotations.

Modifications

• Replace type X = Y aliases in colormaps.py, colors.py, and data.py with typing.TypeAlias, which is available
  under the project’s existing Python >=3.10 requirement.
• Union types written as A | B | C in the alias bodies are rewritten as Union[A, B, C] for the same reason.
• No functional or runtime behavior changes, types are annotations only.

Checklist

• No new dependencies beyond typing_extensions, which is already a transitive dependency of most scientific Python stacks and is explicitly required by many packages already.