UTC Timestamps constructed from datetime.datetime (with datetime.timezone.utc) cannot be compared to pandas Timestamps
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import datetime
ts_to_datetime = pd.to_datetime(datetime.datetime(2018, 3, 8, 16, 58, 41, 182221, tzinfo=datetime.timezone.utc))

ts_utcnow = pd.Timestamp.utcnow()

ts_timestamp = pd.Timestamp(datetime.datetime(2018, 3, 8, 16, 58, 41, 182221, tzinfo=datetime.timezone.utc))

ts_to_datetime - ts_utcnow  # Timestamp subtraction must have the same timezones or no timezones
ts_timestamp - ts_utcnow # same error
ts_to_datetime - ts_timestamp # ok 
```
#### Problem description
Timestamps constructed from `datatime.datetime` objects with UTC timezone have `tz` that is type `datetime.timezone.utc` but `pd.Timestamp.utwnow()` have `tz` that is `pytz.UTC`.


#### Expected Output
``` Python
In [2]: ts_to_datetime - ts_utcnow
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-5564b4c82b42> in <module>()
----> 1 ts_to_datetime - ts_utcnow

pandas/_libs/tslibs/timestamps.pyx in pandas._libs.tslibs.timestamps._Timestamp.__sub__()

TypeError: Timestamp subtraction must have the same timezones or no timezones


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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.3
pytest: 3.0.5
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.13.3
scipy: 0.19.1
pyarrow: 0.7.1
xarray: 0.9.6
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.0.2
openpyxl: 2.4.1
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: 0.9999999
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
