MAINT: Remove tm.assertNotIsInstance from testing
Title is self-explanatory.

Partially addresses #15990.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=h1) Report
> Merging [#16061](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1b52b1218e973382ea958084c47c3f33eb0ed40c?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16061/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16061      +/-   ##
==========================================
+ Coverage   90.82%   90.84%   +0.02%     
==========================================
  Files         159      159              
  Lines       50794    50791       -3     
==========================================
+ Hits        46134    46143       +9     
+ Misses       4660     4648      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.36% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.76% <ø> (-0.06%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.35% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16061?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/16061?src=pr&el=footer). Last update [1b52b12...762dde9](https://codecov.io/gh/pandas-dev/pandas/pull/16061?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!

keem em coming!