BUG: Regression in PeriodIndex.loc string slicing behaviour between 1.0.x and >= 1.1.0
- [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.

- [x] (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

import pandas as pd
print(pd.__version__)
idx = pd.PeriodIndex(['2020-01-01 11:00', '2020-01-01 12:00'], freq='H')
df = pd.DataFrame([1,2], index=idx)
df.loc['2020-01-01 11:00']

```

#### Problem description
It seems there has been a change in the behaviour of string slicing on a `PeriodIndex` in Pandas 1.1 onwards. Despite the exact value being present in the index, a `KeyError` is returned.  

Output in 0.25.3:

```
0    1
Name: 2020-01-01 11:00, dtype: int64
```

Output in 1.0.1
```
0    1
Name: 2020-01-01 11:00, dtype: int6
```

Output in versions >= 1.1.0
```
KeyError: '2020-01-01 11:00'
```

Interestingly, 

```
df.loc['2020-01-01 11']
```

still works and gives the expected answer.

Looks to be due to this assertion: https://github.com/pandas-dev/pandas/blob/8d2a57ecdbe2e45929605763c66cbfd8ec399e19/pandas/core/indexes/period.py#L581 

#### Expected Output
```
0    1
Name: 2020-01-01 11:00, dtype: int6
```


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

<details>

INSTALLED VERSIONS
------------------
commit           : 7d32926db8f7541c356066dcadabf854487738de
python           : 3.7.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.128-microsoft-standard
Version          : #1 SMP Tue Jun 23 12:58:10 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.2
numpy            : 1.19.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 52.0.0.post20210125
Cython           : 0.29.22
pytest           : 6.2.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : 0.4.1
xlsxwriter       : None
lxml.etree       : 4.6.2
html5lib         : 1.1
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.3
IPython          : 7.20.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : 2.7.2
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.1
sqlalchemy       : 1.3.15
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : None

</details>
