Appending to DataFrame removes the timezone-awareness
#### Code Sample, a copy-pastable example if possible

```python
import pytz

import pandas as pd

df = pd.DataFrame(
    {"a": [pd.Timestamp("2019-07-19 07:04:57+0100", tz=pytz.FixedOffset(60))]}
)
df.append(df.iloc[0])
```
#### Problem description

This returns:
```
|    | a                         |
|---:|:--------------------------|
|  0 | 2019-07-19 07:04:57+01:00 |
|  0 | 2019-07-19 06:04:57       |
```
However, I was expecting
```
|    | a                         |
|---:|:--------------------------|
|  0 | 2019-07-19 07:04:57+01:00 |
|  0 | 2019-07-19 07:04:57+01:00 |
```
which indeed is what happens if I instead write
```
df.append(df)
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-72-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 41.6.0.post20191030
Cython           : 0.29.14
pytest           : 5.3.1
hypothesis       : None
sphinx           : 2.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.9.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
s3fs             : 0.4.0
scipy            : 1.3.3
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
