BUG: raise error when setting cached properties

[jorisvandenbossche]

A bug I introduced myself some time ago in #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.

[jreback]

lgtm.

[codecov]

Codecov Report

Merging #20487 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            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%)	⬆️
#single	41.88% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	66.81% <0%> (+1.73%)	⬆️

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