BUG: Timestamp.to_period() fails for freq="MS"
- [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
idx = pd.date_range(start='1/1/2018', end='12/1/2018', freq="MS")
idx[-1].to_period()
```

#### Problem description
This results in the following exception:
```python
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
pandas\_libs\tslibs\period.pyx in pandas._libs.tslibs.period.freq_to_dtype_code()

AttributeError: 'pandas._libs.tslibs.offsets.MonthBegin' object has no attribute '_period_dtype_code'

The above exception was the direct cause of the following exception:

ValueError                                Traceback (most recent call last)
<ipython-input-9-8bcd22fce921> in <module>
      1 idx = pd.date_range(start='1/1/2018', end='12/1/2018', freq="MS")
----> 2 idx[-1].to_period()

pandas\_libs\tslibs\timestamps.pyx in pandas._libs.tslibs.timestamps._Timestamp.to_period()

pandas\_libs\tslibs\period.pyx in pandas._libs.tslibs.period.Period.__new__()

pandas\_libs\tslibs\period.pyx in pandas._libs.tslibs.period.freq_to_dtype_code()

ValueError: Invalid frequency: {0}
```
`Timestamp.to_period()` does not work for `freq="MS"`, other `freq` values might also not work?

#### Expected Output
```python
Period('2018-01', 'M')
```
This can only be achieved by the following workaround:
```python
idx = pd.date_range(start='1/1/2018 00:00:00', end='1/5/2018 00:01:00', freq="MS")
cutoff = idx[-1]
date = pd.DatetimeIndex([cutoff], freq=cutoff.freq)
date.to_period()[0]
>>> Period('2018-01', 'M')
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.8.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : AMD64 Family 21 Model 112 Stepping 0, AuthenticAMD
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.1.5
numpy            : 1.19.4
pytz             : 2020.4
dateutil         : 2.8.1
pip              : 20.2.4
setuptools       : 50.3.1.post20201107
Cython           : 0.29.21
pytest           : 3.6.0
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 0.8.4
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.4
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.50.1

</details>
