json_normalize: Make code more pythonic and avoid modification of meta if mutable
- [x] closes itself #18610 
- [x] tests added / passed
- [x] passes `git diff master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

- Update json_normalize to prevent modifying `meta` parameter.
- Make the code more Pythonic too.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=h1) Report
> Merging [#18610](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0e168188811677f9de72a6a5b97253e551b6b04a?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18610/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18610      +/-   ##
==========================================
- Coverage   91.46%   91.45%   -0.02%     
==========================================
  Files         157      157              
  Lines       51439    51447       +8     
==========================================
- Hits        47051    47049       -2     
- Misses       4388     4398      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.32% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.6% <0%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/pull/18610/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `96.87% <100%> (-0.07%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18610/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18610/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/18610/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.55% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18610/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.28% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=footer). Last update [0e16818...79892a0](https://codecov.io/gh/pandas-dev/pandas/pull/18610?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback is it OK now? Thanks. thanks!