Fix wrong type checking in concat
While the documentation of ``pandas.DataFrame.concat`` specifies that
any mapping of ``Label`` to ``FrameOrSeries`` is acceptable as the first
argument, but the code actually checks for an instance of ``dict``.
This changeset fixes that restriction.

- [x] closes #32863 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
