MAINT: Remove vestigial self.assert*
Remove all remaining `self.assert*` method calls originating from `unittest`.  Any that are left are calls to methods directly defined in the test class or a higher derived `pandas` test class.

Partially addresses #15990.

Once this PR is merged, it is important that we remain vigilant about requiring PR's to adhere to the `pytest` idiom until we remove our dependency on `unittest.TestCase` (after which the builds will do the work when tests fail).
After this PR, all uses of the `unittest.TestCase` methods will have been removed.  The question remains on how to best remove the dependency.  Initially, I was thinking that we should remove `tm.TestCase` entirely, but I see that the `setUpClass` method has a useful property setting call for chained assignment.  Several classes also call the setup and teardown methods here as part of their implementations.

Thus, I am inclined to leave `tm.TestCase` alone and just rename the setup and teardown functions according to `pytest` idiom (see <a href="https://docs.pytest.org/en/2.7.3/xunit_setup.html">here</a>) and just have `tm.TestCase` inherit from `object` instead of `unittest.TestCase`.  Thoughts? is it worthwhile to add NotImplementedError to TestCase for the unittest functions (temporarily)? It's not public API, so I don't see why we should?  The only people who will be "surprised" should be any developers on this library I would think. not sure what u mean

this is a simple way of preventing unittest idioms from slipping back in (until TestCase is fixed) Well, I'm going to fix it as soon as this PR is merged, so that should be moot? :smile: # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16190?src=pr&el=h1) Report
> Merging [#16190](https://codecov.io/gh/pandas-dev/pandas/pull/16190?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/217864e3902592b9723a1070f1bbbb203a1759f4?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16190/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16190?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16190      +/-   ##
==========================================
+ Coverage   90.86%   90.86%   +<.01%     
==========================================
  Files         162      162              
  Lines       50862    50862              
==========================================
+ Hits        46215    46216       +1     
+ Misses       4647     4646       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <ø> (ø)` | :arrow_up: |
| #single | `40.3% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16190?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16190?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16190?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/16190?src=pr&el=footer). Last update [217864e...f52df83](https://codecov.io/gh/pandas-dev/pandas/pull/16190?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Everything is green and ready to merge. thanks!