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

Partially addresses #15990.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=h1) Report
> Merging [#16105](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/60a926bf7f2727a7ba1ff31ac371410662886793?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16105/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16105      +/-   ##
==========================================
- Coverage   90.86%   90.84%   -0.03%     
==========================================
  Files         159      159              
  Lines       50773    50770       -3     
==========================================
- Hits        46136    46121      -15     
- Misses       4637     4649      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.34% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.13% <ø> (-0.06%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.54% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16105?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/16105?src=pr&el=footer). Last update [60a926b...2a1f1a8](https://codecov.io/gh/pandas-dev/pandas/pull/16105?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/16105?src=pr&el=h1) Report
> Merging [#16105](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/60a926bf7f2727a7ba1ff31ac371410662886793?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16105/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16105      +/-   ##
==========================================
- Coverage   90.86%   90.84%   -0.03%     
==========================================
  Files         159      159              
  Lines       50773    50770       -3     
==========================================
- Hits        46136    46121      -15     
- Misses       4637     4649      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.34% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.13% <ø> (-0.06%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.54% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16105?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/16105?src=pr&el=footer). Last update [60a926b...2a1f1a8](https://codecov.io/gh/pandas-dev/pandas/pull/16105?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!