BLD: Fix openpyxl to 2.5.5
`2.5.5` --> `2.5.6` broke compatibility with pandas `Timestamp` objects.

Closes #22595.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22601?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@a5fe9cf`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22601/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22601?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22601   +/-   ##
=========================================
  Coverage          ?   92.04%           
=========================================
  Files             ?      169           
  Lines             ?    50787           
  Branches          ?        0           
=========================================
  Hits              ?    46746           
  Misses            ?     4041           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (?)` | |
| #single | `42.29% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22601?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/22601?src=pr&el=footer). Last update [a5fe9cf...8a1614c](https://codecov.io/gh/pandas-dev/pandas/pull/22601?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!