BUG: `reorder_categories` with `inplace=True` is not changing the `dtype.categories`
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '1.3.2'

In [3]: sr = pd.Series(['a', 'b', 'c'], dtype='category')

In [4]: sr
Out[4]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['a', 'b', 'c']

In [5]: new = sr.cat.reorder_categories(['c', 'b', 'a'])

In [6]: new
Out[6]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['c', 'b', 'a']

In [7]: new.dtype.categories
Out[7]: Index(['c', 'b', 'a'], dtype='object')

In [8]: sr.cat.reorder_categories(['c', 'b', 'a'], inplace=True)
pandas/core/arrays/categorical.py:2630: FutureWarning: The `inplace` parameter in pandas.Categorical.reorder_categories is deprecated and will be removed in a future version. Removing unused categories will always return a new Categorical object.
  res = method(*args, **kwargs)

In [9]: sr
Out[9]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['c', 'b', 'a']

In [10]: sr.dtype.categories
Out[10]: Index(['a', 'b', 'c'], dtype='object')
```

#### Problem description

When we perform `reorder_categories` operation as an inplace op,  the `categories` attribute of `CategoricalDtype` is not updated.

#### Expected Output
```python
In [7]: sr.dtype.categories
Out[7]: Index(['c', 'b', 'a'], dtype='object')
```

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit           : 5f648bf1706dd75a9ca0d29f26eadfbb595fe52b
python           : 3.8.10.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.11.0-27-generic
Version          : #29~20.04.1-Ubuntu SMP Wed Aug 11 15:58:17 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.2
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.4
setuptools       : 57.4.0
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : 6.15.0
sphinx           : 4.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.26.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.53.1



</details>
