TST: .timestamp() uses local timezone
#### Code Sample, a copy-pastable example if possible

```shell
~$ ./test_fast.sh 

=========== FAILURES ============
____ TestTimestamp.test_timestamp ____
[gw0] linux -- Python 3.6.3 /home/kris/.virtualenvs/pandas3/bin/python3.6

self = <pandas.tests.scalar.test_timestamp.TestTimestamp object at 0x7f4a725d2908>

    def test_timestamp(self):
        # GH#17329
        # tz-naive --> treat it as if it were UTC for purposes of timestamp()
        ts = Timestamp.now()
        uts = ts.replace(tzinfo=utc)
        assert ts.timestamp() == uts.timestamp()
    
        tsc = Timestamp('2014-10-11 11:00:01.12345678', tz='US/Central')
        utsc = tsc.tz_convert('UTC')
        # utsc is a different representation of the same time
        assert tsc.timestamp() == utsc.timestamp()
    
        if PY3:
            # should agree with datetime.timestamp method
            dt = ts.to_pydatetime()
>           assert dt.timestamp() == ts.timestamp()
E           AssertionError: assert 1509380391.670673 == 1509383991.670673
E            +  where 1509380391.670673 = <built-in method timestamp of datetime.datetime object at 0x7f4a728bee90>()
E            +    where <built-in method timestamp of datetime.datetime object at 0x7f4a728bee90> = datetime.datetime(2017, 10, 30, 17, 19, 51, 670673).timestamp
E            +  and   1509383991.670673 = <built-in method timestamp of Timestamp object at 0x7f4a7288fde0>()
E            +    where <built-in method timestamp of Timestamp object at 0x7f4a7288fde0> = Timestamp('2017-10-30 17:19:51.670673').timestamp

pandas/tests/scalar/test_timestamp.py:1086: AssertionError
____ TestTimeZones.test_replace_tzinfo ____
[gw0] linux -- Python 3.6.3 /home/kris/.virtualenvs/pandas3/bin/python3.6

self = <pandas.tests.tseries.test_timezones.TestTimeZones object at 0x7f4a71ab5400>

    def test_replace_tzinfo(self):
        # GH 15683
        dt = datetime(2016, 3, 27, 1)
        tzinfo = pytz.timezone('CET').localize(dt, is_dst=False).tzinfo
    
        result_dt = dt.replace(tzinfo=tzinfo)
        result_pd = Timestamp(dt).replace(tzinfo=tzinfo)
    
        if hasattr(result_dt, 'timestamp'):  # New method in Py 3.3
            assert result_dt.timestamp() == result_pd.timestamp()
        assert result_dt == result_pd
        assert result_dt == result_pd.to_pydatetime()
    
        result_dt = dt.replace(tzinfo=tzinfo).replace(tzinfo=None)
        result_pd = Timestamp(dt).replace(tzinfo=tzinfo).replace(tzinfo=None)
    
        if hasattr(result_dt, 'timestamp'):  # New method in Py 3.3
>           assert result_dt.timestamp() == result_pd.timestamp()
E           AssertionError: assert 1459036800.0 == 1459040400.0
E            +  where 1459036800.0 = <built-in method timestamp of datetime.datetime object at 0x7f4a71aa9d50>()
E            +    where <built-in method timestamp of datetime.datetime object at 0x7f4a71aa9d50> = datetime.datetime(2016, 3, 27, 1, 0).timestamp
E            +  and   1459040400.0 = <built-in method timestamp of Timestamp object at 0x7f4a71fd9840>()
E            +    where <built-in method timestamp of Timestamp object at 0x7f4a71fd9840> = Timestamp('2016-03-27 01:00:00').timestamp

pandas/tests/tseries/test_timezones.py:1290: AssertionError
===== 2 failed, 13366 passed, 1777 skipped, 10 xfailed, 1 xpassed in 111.18 seconds =====
```
#### Problem description
On `python3.5` and `python3.6` unittests fail as shown above.  
Same results via `pytest pandas` and `./test_fast.sh`.

#### Expected Output

Passing unittests.

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

<details>

INSTALLED VERSIONS
------------------
commit: 8449ffd90eb07abaa76b5c17e2a7fa753dd881f8
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-37-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0rc1+67.g8449ffd
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.6.0
Cython: None
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.5
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>