MAINT: Remove assertIsInstance from testing
Title is self-explanatory.

Partially addresses #15990.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=h1) Report
> Merging [#16081](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ff00b55de28ed12fc37ebeabfc2a748821542227?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16081/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16081      +/-   ##
==========================================
- Coverage   90.82%   90.82%   -0.01%     
==========================================
  Files         159      159              
  Lines       50791    50784       -7     
==========================================
- Hits        46132    46124       -8     
- Misses       4659     4660       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.6% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.36% <61.9%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.62% <100%> (-0.14%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16081?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/16081?src=pr&el=footer). Last update [ff00b55...ccf75ae](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=h1) Report
> Merging [#16081](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ff00b55de28ed12fc37ebeabfc2a748821542227?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16081/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16081      +/-   ##
==========================================
- Coverage   90.82%   90.82%   -0.01%     
==========================================
  Files         159      159              
  Lines       50791    50784       -7     
==========================================
- Hits        46132    46124       -8     
- Misses       4659     4660       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.6% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.36% <61.9%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.62% <100%> (-0.14%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16081?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/16081?src=pr&el=footer). Last update [ff00b55...ccf75ae](https://codecov.io/gh/pandas-dev/pandas/pull/16081?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!