BUG: Period constructor does not take into account nanonseconds
- [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.
---

#### Code Sample, a copy-pastable example:
```python
print(pd.Period("2000-12-31 23:59:59.000000999").to_timestamp().nanosecond)
print(pd.Period("2000-12-31 23:59:59.000000500").to_timestamp().nanosecond)
print(pd.Period("2000-12-31 23:59:59.000000001").to_timestamp().nanosecond)
print(pd.Period("2000-12-31 23:59:59.000000001").to_timestamp(freq='ns').nanosecond)
```
output:
```python
0
0
0
0
```

#### Problem description

Nanoseconds are set to `0` when using `to_timestamp`  function.

#### Expected Output
```python
999
500
1
1
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : c71bfc36211b5e2d860a06d8fbef902b757bd6e4
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-33-generic
Version          : #37-Ubuntu SMP Thu May 21 12:53:59 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 1.1.0.dev0+1767.gc71bfc362
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.7.3
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : 5.4.2
hypothesis       : 5.16.0
sphinx           : 3.0.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.17
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
