STYLE: Use decorator syntax instead of legacy syntax for defining properties in Cython
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

See the [Cython docs](http://cython.readthedocs.io/en/latest/src/userguide/extension_types.html#properties) for a comparison of the decorator syntax and legacy syntax.  I only found a few instances of the legacy syntax; looks like most of the Cython code is already using decorator syntax.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=h1) Report
> Merging [#18602](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0e168188811677f9de72a6a5b97253e551b6b04a?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/18602/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18602      +/-   ##
==========================================
- Coverage   91.46%   91.45%   -0.02%     
==========================================
  Files         157      157              
  Lines       51439    51439              
==========================================
- Hits        47051    47042       -9     
- Misses       4388     4397       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.32% <ø> (ø)` | :arrow_up: |
| #single | `40.6% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18602/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18602/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18602?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/18602?src=pr&el=footer). Last update [0e16818...501e9ce](https://codecov.io/gh/pandas-dev/pandas/pull/18602?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/18602?src=pr&el=h1) Report
> Merging [#18602](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a7646638d06f1ce98481b88f3505e2b4badf172c?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/18602/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18602      +/-   ##
==========================================
- Coverage   91.59%   91.58%   -0.02%     
==========================================
  Files         155      155              
  Lines       51255    51255              
==========================================
- Hits        46948    46941       -7     
- Misses       4307     4314       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.44% <ø> (ø)` | :arrow_up: |
| #single | `40.67% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18602/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18602/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18602/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.01% <0%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18602?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/18602?src=pr&el=footer). Last update [a764663...6b136cf](https://codecov.io/gh/pandas-dev/pandas/pull/18602?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 so the ``.values`` value is actually set *only* via a c call, IOW this is not a python settable propery > so the .values value is actually set only via a c call, IOW this is not a python settable propery

This 100% makes sense.  Regardless, the property is almost certainly not needed. Would it be worthwhile to add a check for legacy syntax to `lint.sh`?  Or is this something that's infrequent enough to not bother with? i don’t think necessary to add a lint check here 
 Green after rebase to include the Cython version bump.  Didn't need to add a setter or any additional code. thanks!