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

also pls add a test for this (in the OP) Hello @pbourguignon! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-03-24 17:07:11 UTC One last question: should I add an entry to the existing whatsnew for 1.1.0? yes closing in favor of #32953  @pbourguignon you have 2 PR for the same issue;. I closed tihs one as the other is more recent (and has the changes here too). If this is not correct pls comments. > @pbourguignon you have 2 PR for the same issue;. I closed tihs one as the other is more recent (and has the changes here too). If this is not correct pls comments.

That's alright, I just couldn't find how to create the later PR against the earlier one.