STYLE: fix pylint redefined-oute-name warnings (pandas-dev#49656)
Fixed warnings for the following files :-
- pandas/core/internals/concat.py
- pandas/core/reshape/merge.py
- pandas/io/json/_json.py
you'll need to run pre-commit checks (see the contributing guide for how to do that), but apart from that looks good closing as stale, thanks anyway for your PR!