CLN: remove F821 flake8 error in test case
Kill flake8 error: F821 undefined name 'unicode'
`compat.text_type == unicode if PY2`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22009?src=pr&el=h1) Report
> Merging [#22009](https://codecov.io/gh/pandas-dev/pandas/pull/22009?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/322dbf4f812f47d4eaee5d8afa10e17acf6dbc67?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22009/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22009?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22009   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
=======================================
  Files         167      167           
  Lines       50578    50578           
=======================================
  Hits        46530    46530           
  Misses       4048     4048
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <ø> (ø)` | :arrow_up: |
| #single | `42.17% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22009?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/22009?src=pr&el=footer). Last update [322dbf4...9a00a35](https://codecov.io/gh/pandas-dev/pandas/pull/22009?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @holymonson Thanks!