Allow choosing the utc timezone class in pd.to_datetime
#### Code Sample, a copy-pastable example if possible

```python
from datetime import datetime, timezone
import pandas as pd

dt1 = pd.to_datetime(datetime(2020, 3, 11), utc=True)
print(repr(dt1))
print(type(dt1.tz))

dt2 = pd.to_datetime(datetime(2020, 3, 11, tzinfo=timezone.utc))
print(repr(dt2))
print(type(dt2.tz))

# dragons here
print(dt1 - dt2)
```

outputs

```
Timestamp('2020-03-11 00:00:00+0000', tz='UTC')
<class 'pytz.UTC'>
Timestamp('2020-03-11 00:00:00+0000', tz='UTC')
<class 'datetime.timezone'>

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

#### Problem description

There is no ability to specify which "UTC" the Timestamp should be. I suggest extending the interface of `pd.to_datetime()` to specify `utc_cls=pytz.UTC`.

#### Expected Output

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

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-40-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.17.4
pytz             : 2019.2
dateutil         : 2.7.3
pip              : 19.3.1
setuptools       : 42.0.1
Cython           : 0.29.14
pytest           : 5.3.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.10.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.3.1
pyxlsb           : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.3.12
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
```

</details>
