pd.Timestamp.timestamp() doesn't match stdlib
edit: See https://github.com/pandas-dev/pandas/issues/29779#issuecomment-557747641

I searched for this among open issues but couldn't find it. Apologies in advance if I overlooked an existing issue, resolved or otherwise.

```python
Python 3.7.4 (default, Sep  7 2019, 18:29:04)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> import pandas as pd
>>> dt = datetime.datetime(1970, 1, 1)
>>> pdt = pd.Timestamp(1970, 1, 1)
>>> dt.timestamp() == pdt.timestamp()  # I expected this to be True
False
>>> dt.timestamp()  # I am in PST, hence the 8-hour offset from 0
28800.0
>>> pdt.timestamp()
0.0
```


[Python3 docs explain](https://docs.python.org/3/library/datetime.html#datetime.datetime.timestamp) that datetime.datetime.timestamp() assume local time if no tzinfo is specified: "Naive datetime instances are assumed to represent local time and this method relies on the platform C mktime() function to perform the conversion."


#### Problem description
I expected the behavior of pd.Timestamp.timestamp to match that of the stdlib (datetime.datetime.stdlib). Instead, pd.Timestamp.timestamp appears to treat naive timestamps differently.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 40.8.0
Cython           : 0.29.13
pytest           : 5.2.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.3 (dt dec pq3 ext lo64)
jinja2           : 2.10.1
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.8
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
