BUG: groupby.agg returns incorrect results for uint64 cols (#26310)
- [x] closes #26310
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A way to avoid incorrect coercion to float64 in the case of uint64 input is to introduce an additional check to `ensure_int64_or_float64`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=h1) Report
> Merging [#26359](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4e4f5bd8fbd228b08ac8b331a6435c9f02230184?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26359/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26359      +/-   ##
==========================================
- Coverage   92.04%   92.03%   -0.01%     
==========================================
  Files         175      175              
  Lines       52289    52292       +3     
==========================================
- Hits        48130    48129       -1     
- Misses       4159     4163       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <100%> (ø)` | :arrow_up: |
| #single | `40.7% <0%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `97.43% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.01% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26359?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/26359?src=pr&el=footer). Last update [4e4f5bd...2e9ca6b](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=h1) Report
> Merging [#26359](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1263e1a9cac179f453aa6223f00cbaa3e158a878?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26359/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26359      +/-   ##
==========================================
- Coverage   91.73%   91.72%   -0.01%     
==========================================
  Files         174      174              
  Lines       50741    50746       +5     
==========================================
+ Hits        46548    46549       +1     
- Misses       4193     4197       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (ø)` | :arrow_up: |
| #single | `41.7% <28.57%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `97.45% <100%> (+0.04%)` | :arrow_up: |
| [pandas/core/groupby/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9vcHMucHk=) | `95.96% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26359/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26359?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/26359?src=pr&el=footer). Last update [1263e1a...ce22d54](https://codecov.io/gh/pandas-dev/pandas/pull/26359?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @mahepe! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-05-16 18:40:25 UTC all green @jreback ! thanks @mahepe 

