BUG: raise error when setting cached properties
A bug I introduced myself some time ago in https://github.com/pandas-dev/pandas/pull/19991. Apparently the object needs to explicitly raise in the `__set__` method, otherwise the property is assumed to be settable:

```
In [6]: idx = pd.Index([1, 2, 3])

In [7]: idx.is_unique
Out[7]: True

In [8]: idx.is_unique = False

In [9]: idx.is_unique
Out[9]: False

In [10]: idx.is_monotonic
Out[10]: True

In [11]: idx.is_monotonic = False
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-11-b2a74ac7466c> in <module>()
----> 1 idx.is_monotonic = False

AttributeError: can't set attribute
```

`is_unique` is a cached property, didn't raise anymore on master (see above), in contrast to normal property `is_monotonic`.

This patch ensures also cached properties have the same error.
lgtm. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20487?src=pr&el=h1) Report
> Merging [#20487](https://codecov.io/gh/pandas-dev/pandas/pull/20487?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6c0c27730fb5b20978824fb380449cd3acef55f7?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20487/graphs/tree.svg?height=150&token=eZ4WkYLtcO&src=pr&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20487?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20487      +/-   ##
==========================================
+ Coverage   91.82%   91.85%   +0.02%     
==========================================
  Files         152      152              
  Lines       49235    49235              
==========================================
+ Hits        45212    45224      +12     
+ Misses       4023     4011      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.24% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.88% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20487?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20487/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20487?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/20487?src=pr&el=footer). Last update [6c0c277...7d626fb](https://codecov.io/gh/pandas-dev/pandas/pull/20487?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
