Indexing into a series with multi-index containing periods raises an exception
#### Code Sample, a copy-pastable example if possible

```python
import pandas
tuples = [(pandas.Period('2018-01-01', freq='D'), 1)]
index = pandas.MultiIndex.from_tuples(tuples)
s = pandas.Series([1.0], index=index)
print(s[~s.isnull()])
```
#### Problem description
Running the code above causes an exception with a long traceback:
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item()

TypeError: an integer is required

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/indexes/period.py in get_loc(self, key, method, tolerance)
    881         try:
--> 882             return self._engine.get_loc(key)
    883         except KeyError:

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

KeyError: '__next__'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
pandas/_libs/tslibs/parsing.pyx in pandas._libs.tslibs.parsing.parse_datetime_string_with_reso()

pandas/_libs/tslibs/parsing.pyx in pandas._libs.tslibs.parsing.dateutil_parse()

ValueError: Unknown datetime string format, unable to parse: __next__

During handling of the above exception, another exception occurred:

DateParseError                            Traceback (most recent call last)
<ipython-input-9-81179fad4d42> in <module>()
      3 index = pandas.MultiIndex.from_tuples(tuples)
      4 s = pandas.Series([1.0], index=index)
----> 5 print(s[~s.isnull()])

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/series.py in __getitem__(self, key)
    802             raise
    803 
--> 804         if is_iterator(key):
    805             key = list(key)
    806 

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/dtypes/inference.py in is_iterator(obj)
    153         # Python 3 generators have
    154         # __next__ instead of next
--> 155         return hasattr(obj, '__next__')
    156 
    157 

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/generic.py in __getattr__(self, name)
   4372             return object.__getattribute__(self, name)
   4373         else:
-> 4374             if self._info_axis._can_hold_identifiers_and_holds_name(name):
   4375                 return self[name]
   4376             return object.__getattribute__(self, name)

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/indexes/base.py in _can_hold_identifiers_and_holds_name(self, name)
   2109         """
   2110         if self.is_object() or self.is_categorical():
-> 2111             return name in self
   2112         return False
   2113 

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/indexes/multi.py in __contains__(self, key)
    547         hash(key)
    548         try:
--> 549             self.get_loc(key)
    550             return True
    551         except (LookupError, TypeError):

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/indexes/multi.py in get_loc(self, key, method)
   2235 
   2236         if not isinstance(key, tuple):
-> 2237             loc = self._get_level_indexer(key, level=0)
   2238 
   2239             # _get_level_indexer returns an empty slice if the key has

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/indexes/multi.py in _get_level_indexer(self, key, level, indexer)
   2494         else:
   2495 
-> 2496             loc = level_index.get_loc(key)
   2497             if isinstance(loc, slice):
   2498                 return loc

~/.virtualenvs/shackleton3/lib/python3.7/site-packages/pandas/core/indexes/period.py in get_loc(self, key, method, tolerance)
    886 
    887             try:
--> 888                 asdt, parsed, reso = parse_time_string(key, self.freq)
    889                 key = asdt
    890             except TypeError:

pandas/_libs/tslibs/parsing.pyx in pandas._libs.tslibs.parsing.parse_time_string()

pandas/_libs/tslibs/parsing.pyx in pandas._libs.tslibs.parsing.parse_datetime_string_with_reso()

DateParseError: Unknown datetime string format, unable to parse: __next__
```
This appears to happen only if the series has a multi-index that contains periods as one of its levels. Datetimes won't cause it, for instance.

The `s[~s.isnull()]` is equivalent to `s.dropna()`, which does work.

Neither `~s.isnull()` nor `s[[True]]` cause the exception by themselves. It appears to take the combination.

#### Expected Output
```
2018-01-01  1    1.0
dtype: float64
```
#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.8.0
pip: 18.0
setuptools: 40.2.0
Cython: 0.28.5
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.5.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 2.2.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.11
pymysql: 0.9.2
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
