COMPAT: Pypy tweaks
a set of tweaks that I discovered when getting PyPY to pass tests, some could cause issues on CPython as well:

- 496cc3a clears up an assumption that sets are sorted
- d161b08 makes sure PyList_GET_SIZE is used only on PyListObject, if used on an ndarray it will return the value of the data pointer, which is not the intention at all
- 2744c5c separates the cases where ``float('nan') is not np.nan`` and like cases by implementation, on PyPy the ``is`` comparison always checks the value of struct.pack('d', val) for equality.
- 5425137 cleans up the use of ``rank`` and ``mean`` as keys (the warnings are ignored?) and adds a trivial assert where a test was simply making a call

I left these as seperate commits for ease of discussion and possible rejection, if desired I can squash them to a single commit

Just for my understanding on 2744c5c , you're saying this would be true on PyPy?
```python
>>> a = 1.2
>>> b = 1.2
>>> a is b
False
``` @chris-b1 about is-ness, yes that is True
```
>>>> a = float('nan')
>>>> b = float('nan')
>>>> a is b
True
>>>> a=1.2
>>>> b=1.2
>>>> a is b
True
```
on cpython both are False # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=h1) Report
> Merging [#17351](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6bab9d18bef3b7fccab2830d6dad78d0fb476ed8?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17351/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17351      +/-   ##
==========================================
- Coverage   91.03%   90.99%   -0.05%     
==========================================
  Files         162      162              
  Lines       49567    49568       +1     
==========================================
- Hits        45125    45105      -20     
- Misses       4442     4463      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.24% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17351?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/17351?src=pr&el=footer). Last update [6bab9d1...b1c79c4](https://codecov.io/gh/pandas-dev/pandas/pull/17351?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/17351?src=pr&el=h1) Report
> Merging [#17351](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d4577911c750f2f48f760ce451d413116bed72da?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17351/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17351      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49587    49588       +1     
==========================================
- Hits        45203    45195       -8     
- Misses       4384     4393       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <ø> (ø)` | :arrow_up: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17351?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/17351?src=pr&el=footer). Last update [d457791...bb7ef63](https://codecov.io/gh/pandas-dev/pandas/pull/17351?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @mattip! Thanks for updating the PR.









Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on September 06, 2017 at 18:50 Hours UTC @jreback ping, test committed in 2f653f1
 thanks @mattip always happy to have patches from pypy! I don't think this had any issues to close?