Rename Z alias in histogram

[michaelgrund]

Description of proposed changes

As discussed in #1272 the alias for Z should be renamed since type is already used as Python built-in method.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

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[willschlitzer]

Good catch @seisman and @michaelgrund. I think this is a smart change.

[weiji14]

Nice, feel free to merge this in directly.