TST: Remove check_exact from assert_almost_equal
From a semantic point of view, if you're checking for approximate equality, there shouldn't be a parameter that allows you to disable the whole purpose of the function.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21756?src=pr&el=h1) Report
> Merging [#21756](https://codecov.io/gh/pandas-dev/pandas/pull/21756?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/04caa569e5316e1a6526f0c78142a677ca4b2ee9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21756/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21756?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21756   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files         159      159           
  Lines       49733    49733           
=======================================
  Hits        45721    45721           
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <100%> (ø)` | :arrow_up: |
| #single | `42% <66.66%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21756?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21756/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.33% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21756?src=pr&el=footer). Last update [04caa56...6b220bd](https://codecov.io/gh/pandas-dev/pandas/pull/21756?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
