FIX: define `DataFrame.items` for all versions of python
Closes #13918, #17213

This leaves a slight semantic difference between `dict.items` and
`DateFrame.items` in python2, however there is no code currently
expecting `DataFrame.items` to return a list and in python3 there is
no `dict.iteritems`.

This eases writing 'native' python3 code that also runs under python2.

Do you want a whatsnew for this?

- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

prior PR https://github.com/pandas-dev/pandas/pull/13920 (closed as stale) I disagree with @jreback from a year ago that `items` should be not-lazy in python2.  We are a year closer to python2 EOL and we should not jump through hoops to maintain exactly identical behavior, but not ideal, behavior.

However I have just spent more time complaining about not wanting to do it than it would take to just implement it, so either way is fine with me. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=h1) Report
> Merging [#17214](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f165b90ed27487287e1c8a0a6c4e66344b2c731d?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/17214/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17214      +/-   ##
==========================================
- Coverage   91.02%   91.01%   -0.02%     
==========================================
  Files         162      162              
  Lines       49517    49518       +1     
==========================================
- Hits        45075    45067       -8     
- Misses       4442     4451       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.79% <100%> (ø)` | :arrow_up: |
| #single | `40.26% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17214?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/17214?src=pr&el=footer). Last update [f165b90...47947db](https://codecov.io/gh/pandas-dev/pandas/pull/17214?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/17214?src=pr&el=h1) Report
> Merging [#17214](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4e9c0d1f2156c656df5da4ac3f00190f0da5828b?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/17214/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17214      +/-   ##
==========================================
- Coverage      91%   90.98%   -0.02%     
==========================================
  Files         162      162              
  Lines       49565    49567       +2     
==========================================
- Hits        45109    45101       -8     
- Misses       4456     4466      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.24% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.23% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17214?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/17214?src=pr&el=footer). Last update [4e9c0d1...b8a9ab7](https://codecov.io/gh/pandas-dev/pandas/pull/17214?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @tacaswell : I think your change is almost exactly the same as the original PR, which I think people were in agreement with back then.  This looks fine (just needs a `whatsnew`). trivial changes. rebase and ping on green. ping @jreback all green! sigh, just force-pushed a spelling correction to the docs. thanks!