Prevent UnicodeDecodeError in pivot_table under Py2
- [x] closes #13292 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
@mpenkov : Will need a `whatsnew` entry # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=h1) Report
> Merging [#17489](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/23050dca1b404d23527132c0277f3d40dc41cab8?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17489/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17489      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49534    49534              
==========================================
- Hits        45153    45148       -5     
- Misses       4381     4386       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <100%> (ø)` | :arrow_up: |
| #single | `40.22% <50%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.35% <100%> (+0.99%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.05%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17489?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/17489?src=pr&el=footer). Last update [23050dc...e851082](https://codecov.io/gh/pandas-dev/pandas/pull/17489?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/17489?src=pr&el=h1) Report
> Merging [#17489](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/23050dca1b404d23527132c0277f3d40dc41cab8?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17489/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17489      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49534    49534              
==========================================
- Hits        45153    45148       -5     
- Misses       4381     4386       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <100%> (ø)` | :arrow_up: |
| #single | `40.22% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.35% <100%> (+0.99%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.05%)` | :arrow_down: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `85.71% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17489?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/17489?src=pr&el=footer). Last update [23050dc...ce96abd](https://codecov.io/gh/pandas-dev/pandas/pull/17489?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung Added the what's new. Please check. thanks @mpenkov 