BLD: hashtable.pxd is a pxd depends for _join, hashtable, and index
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15218?src=pr) is 85.98% (diff: 100%)
> Merging [#15218](https://codecov.io/gh/pandas-dev/pandas/pull/15218?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #15218   diff @@
==========================================
  Files           140        140          
  Lines         51272      51272          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          44087      44086     -1   
- Misses         7185       7186     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [64d7670...5322245](https://codecov.io/gh/pandas-dev/pandas/compare/64d7670d99a11ec4e263da73f2bb1335cb4290d3...53222457316c2e9a61845fbc7ab33ecb18105b22?src=pr)