ENH: Support dateutil.tz.tzrange objects
#### Code Sample, a copy-pastable example if possible

```python
mytz = dateutil.tz.tzrange("EST", -18000, "EDT", -14400,
                  dateutil.relativedelta.relativedelta(hours=+2, month=4,
                                weekday=dateutil.relativedelta.SU(+1)),
                  dateutil.relativedelta.relativedelta(hours=+1, month=10,
                                weekday=dateutil.relativedelta.SU(-1)))
pd.to_datetime('20190204', utc=True).tz_convert(mytz)
```

> ---------------------------------------------------------------------------
> AttributeError                            Traceback (most recent call last)
> pandas\_libs\tslibs\timezones.pyx in pandas._libs.tslibs.timezones.get_dst_info()
> 
> AttributeError: 'NoneType' object has no attribute 'total_seconds'
> Exception ignored in: 'pandas._libs.tslibs.conversion.localize_tso'
> Traceback (most recent call last):
>   File "pandas\_libs\tslibs\timezones.pyx", line 220, in pandas._libs.tslibs.timezones.get_dst_info
> AttributeError: 'NoneType' object has no attribute 'total_seconds'
> Out[243]: Timestamp('2019-02-04 00:00:00')

#### Problem description

`tz_convert` fails on custom timezone with DST provided. Here is the example with `dateutil` but it would fail with any custom timezone with DST offset

#### Expected Output
`Timestamp('2019-02-03 19:00:00-0500', tz='mytz')`

#### Output of ``pd.show_versions()``
<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 85 Stepping 4, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.1
numpy            : 1.16.5
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.4.0
Cython           : 0.29.13
pytest           : 5.2.1
hypothesis       : None
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.1
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : 0.13.0
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.9
tables           : 3.5.2
xarray           : 0.14.0
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.1

</details>
