DOC: Use correct pandas when building documentation
This manifested itself in #25419 - when trying to build documentation the version of pandas that gets inserted into the globals of this module may have inadvertently been picked up from another install on the system. This prioritizes the version of pandas dictated by command line args
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25486?src=pr&el=h1) Report
> Merging [#25486](https://codecov.io/gh/pandas-dev/pandas/pull/25486?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/db978c716369064421c5ca71bd26002e0021e0d1?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/25486/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25486?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25486      +/-   ##
==========================================
+ Coverage   91.75%   91.75%   +<.01%     
==========================================
  Files         173      173              
  Lines       52955    52955              
==========================================
+ Hits        48588    48589       +1     
+ Misses       4367     4366       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25486?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25486/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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