Merge isinstance calls

[DimitriPapadopoulos]

You can pass a tuple of types you want to check as the second argument to isinstance. Therefore, merge multiple consecutive isinstance calls into one. It is clearer and improves readability.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[joshmoore]

Thanks as always, @DimitriPapadopoulos.