• If a single type is passed in, save an iteration over a tuple.
• Don't compute the type names for the error message unless an error
  actually occurred.

Together, this speeds up _check_isinstance(int, x=1) by more than
2-fold, and _check_isinstance((float, int), x=1) by a few dozen-%.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).