COMPAT: 32-bit indexing compat
xref #19539

cc @toobaz https://travis-ci.org/MacPython/pandas-wheels/jobs/376647689

these are built once a day  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=h1) Report
> Merging [#20989](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/eff1faf2729fa0fb0c2d7913107201e475de33d6?src=pr&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20989/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20989      +/-   ##
==========================================
+ Coverage   91.82%   91.88%   +0.05%     
==========================================
  Files         153      153              
  Lines       49488    49819     +331     
==========================================
+ Hits        45443    45775     +332     
+ Misses       4045     4044       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (+0.06%)` | :arrow_up: |
| #single | `41.99% <ø> (+0.13%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20989/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `97.2% <0%> (+0.55%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=footer). Last update [eff1faf...5ce73e1](https://codecov.io/gh/pandas-dev/pandas/pull/20989?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
