DataFrame.astype does not handle multiple timezone aware Timestamps
```python
import pandas

df = pandas.DataFrame({'a': [pandas.Timestamp('20200309T120000.000000-0400'), pandas.Timestamp('20200309T130000.000000-0400')]})
df.astype({'a': pandas.api.types.DatetimeTZDtype(unit='ns', tz='UTC')})
```
```
                          a
0 2020-03-09 16:00:00+00:00
1 2020-03-09 17:00:00+00:00
```
```python
df = pandas.DataFrame({'a': [pandas.Timestamp('20200309T120000.000000-0400'), pandas.Timestamp('20200309T130000.000000-0500')]})
df.astype({'a': pandas.api.types.DatetimeTZDtype(unit='ns', tz='UTC')})

ValueError: Tz-aware datetime.datetime cannot be converted to datetime64 unless utc=True
```

Similarly, something like `df['a'].dt.tz_convert(tz='utc')` also returns the same error

#### Problem description

Calling `pandas.DataFrame.astype` with a column of mixed timezone aware Timestamps fails to convert to `UTC`

#### Expected Output

I would expect this to return the same result as `df.applymap(lambda x: x.tz_convert(tz='utc'))`

```
0 2020-03-09 16:00:00+00:00
1 2020-03-09 18:00:00+00:00
```

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-28-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            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.0.0.post20200309
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
