CLN: separate out groupby algos
- separate out groupby algorithms to separate lib
- release GIL on median
- release GIL on is_lexsorted / fix memory leak
- release GIL on nancorr
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15775?src=pr&el=h1) Report
> Merging [#15775](https://codecov.io/gh/pandas-dev/pandas/pull/15775?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/32dd92912f15a5c66035f5674c116d23f21bdbca?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15775      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.02%     
==========================================
  Files         143      143              
  Lines       49403    49403              
==========================================
- Hits        44966    44957       -9     
- Misses       4437     4446       +9
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15775?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/compare/32dd92912f15a5c66035f5674c116d23f21bdbca...4e2bfecf9866cf82289fff3fa774247bae3c6f37?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `94.96% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/32dd92912f15a5c66035f5674c116d23f21bdbca...4e2bfecf9866cf82289fff3fa774247bae3c6f37?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/32dd92912f15a5c66035f5674c116d23f21bdbca...4e2bfecf9866cf82289fff3fa774247bae3c6f37?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.86% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/compare/32dd92912f15a5c66035f5674c116d23f21bdbca...4e2bfecf9866cf82289fff3fa774247bae3c6f37?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.29% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/32dd92912f15a5c66035f5674c116d23f21bdbca...4e2bfecf9866cf82289fff3fa774247bae3c6f37?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.3% <0%> (+0.33%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15775?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/15775?src=pr&el=footer). Last update [32dd929...4e2bfec](https://codecov.io/gh/pandas-dev/pandas/compare/32dd92912f15a5c66035f5674c116d23f21bdbca...4e2bfecf9866cf82289fff3fa774247bae3c6f37?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).