BUG: pd.to_datetime does not assign UTC TZ to Unix Epoch Timestamps
- [ X ] I have checked that this issue has not already been reported.

- [ X ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

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

now = datetime.now()

df = pd.DataFrame({'ts_pd': [now.timestamp()], 'ts_py': [now.timestamp()]})
print(f"It is now: {now.timestamp()} seconds from Unix Epoch (00:00:00 UTC on 1 January 1970)")
df['ts_pd'] = pd.to_datetime(df['ts_pd'], unit='s')
df['ts_py'] = df['ts_py'].apply(datetime.fromtimestamp)
print(df)

# It is now: 1620338266.89658 seconds from Unix Epoch (00:00:00 UTC on 1 January 1970)
# ts_pd                           ts_py
# 2021-05-06 21:57:46  2021-05-06 17:57:46
```

#### Problem description
I would expect `to_datetime` to behave the same as `fromtimestamp`. That is, if returning a TZ-naive time, it should probably be  a localtime. If parsing a unix epoch timestamp, we know the value is in UTC, so leaving it TZ-unaware is a footgun as I think a naive user would expect either (1) localtime or (2) to have UTC tzinfo attached to prevent mingling with true localtimes. 

I encountered this problem because I was parsing timestamps from a couple of files. In one file were text-form timestamps, which were parsed as local-time. In the other were epoch-form timestamps, which were parsed as TZ-unaware UTC. If I unconditionally pass utc=True, now my text-form timestamps are mistakenly given UTC tzinfo. In the end I wrote a helper which only passes utc=True if the column is numerical.

This table shows the outputs and workaround:
|    | str                                                 | ts                                  | comment |
|---:|:----------------------------------------------------|:------------------------------------|--|
|  0 | pd.to_datetime(now.isoformat())                     | 2021-05-06 18:03:31.766199          ||
|  1 | datetime.fromtimestamp(now.timestamp())             | 2021-05-06 18:03:31.766199          ||
|  2 | datetime.utcfromtimestamp(now.timestamp())          | 2021-05-06 22:03:31.766199          |This is OK, I specifically asked for UTC not localtime.|
|  3 | pd.to_datetime(now.timestamp(), unit="s")           | 2021-05-06 22:03:31.766199112       |Not OK. I've now parsed a different time than if I had parsed isoformat() of the same time.|
|  4 | pd.to_datetime(now.timestamp(), unit="s", utc=True) | 2021-05-06 22:03:31.766199112+00:00 |OK, but should be default.|

#### Expected Output
| ts_pd | ts_py |
|--------|--------|
| 2021-05-06 21:53:42+00:00 | 2021-05-06 17:53:42|


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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.0.5
numpy            : 1.18.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 49.2.0.post20200714
Cython           : 0.29.21
pytest           : 5.4.3
hypothesis       : None
sphinx           : 3.1.2
blosc            : None
feather          : None
xlsxwriter       : 1.2.9
lxml.etree       : 4.5.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : 1.3.2
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.2
matplotlib       : 3.2.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.4
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.3
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.18
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.9
numba            : 0.50.1
</details>
