MAINT: Remove tm.assertIsNot from testing
Title is self-explanatory.  Also clean up other non-`pytest` idioms where I was removing `assertIsNot`.

Partially addresses #15990.

lgtm. FYI, travis is undergoing maint. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=h1) Report
> Merging [#16024](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a25272ba72741c3e2862f5b0e06279242ffef2b2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16024/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16024      +/-   ##
==========================================
+ Coverage   90.99%   90.99%   +<.01%     
==========================================
  Files         154      154              
  Lines       50471    50472       +1     
==========================================
+ Hits        45926    45927       +1     
  Misses       4545     4545
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <100%> (ø)` | :arrow_up: |
| #single | `40.43% <42.85%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.76% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16024?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/16024?src=pr&el=footer). Last update [a25272b...c678575](https://codecov.io/gh/pandas-dev/pandas/pull/16024?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/16024?src=pr&el=h1) Report
> Merging [#16024](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a25272ba72741c3e2862f5b0e06279242ffef2b2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16024/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16024      +/-   ##
==========================================
+ Coverage   90.99%   90.99%   +<.01%     
==========================================
  Files         154      154              
  Lines       50471    50472       +1     
==========================================
+ Hits        45926    45927       +1     
  Misses       4545     4545
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <100%> (ø)` | :arrow_up: |
| #single | `40.43% <42.85%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.76% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16024?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/16024?src=pr&el=footer). Last update [a25272b...c678575](https://codecov.io/gh/pandas-dev/pandas/pull/16024?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! @jorisvandenbossche : Shoot, it looks there were a few other instances of `assertIsNot` that didn't make the cut.  I'll remove them in a follow-up. And all tests still passed? It's because they're `self.assertIsNot`, not `tm.assetIsNot`