CLN: (re-)enable infer_dtype to catch complex
- [x] takes a small bite out of #23554
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

`complex` should already be inferred according to the docstring, but isn't. I'm hitting this while working on my PR for #23833 so splitting that off as a separate change.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=h1) Report
> Merging [#25382](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f4568fd76e864d8aee3d23f5a81302262d6e0dcb?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/25382/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25382      +/-   ##
==========================================
+ Coverage   91.73%   91.73%   +<.01%     
==========================================
  Files         173      173              
  Lines       52832    52832              
==========================================
+ Hits        48467    48468       +1     
+ Misses       4365     4364       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25382/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25382?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/25382?src=pr&el=footer). Last update [f4568fd...f2b4e9d](https://codecov.io/gh/pandas-dev/pandas/pull/25382?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/25382?src=pr&el=h1) Report
> Merging [#25382](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/54492791116108199c24734a0220560974eb3372?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/25382/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25382   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files         173      173           
  Lines       52845    52845           
=======================================
  Hits        48480    48480           
  Misses       4365     4365
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25382?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/25382?src=pr&el=footer). Last update [5449279...12740f2](https://codecov.io/gh/pandas-dev/pandas/pull/25382?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks