BUG: pytz.exceptions.UnknownTimeZoneError when loading psycopg2 timezone from Parquet
- [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.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import datetime as dt
import pandas as pd
from psycopg2.tz import FixedOffsetTimezone

# Simulate getting a timestamp from the DB
psycopg2_utc = FixedOffsetTimezone()
timestamp = dt.datetime.now(psycopg2_utc)

df = pd.DataFrame({'foo': [timestamp]})

df.to_parquet('test1.parquet')
try:
    pd.read_parquet('test1.parquet')
except Exception as e:
    print(f'First load failed: {e}')

# Fix by converting to pytz UTC
df['foo'] = df['foo'].dt.tz_convert('UTC')

df.to_parquet('test2.parquet')
pd.read_parquet('test2.parquet')
print('Second load succeeded')
```

#### Problem description

When a DataFrame that contains a datetime with a psycopg2 timezone object is stored in a Parquet file then loading the file raises a `pytz.exceptions.UnknownTimeZoneError`. Conversion to pytz UTC via `.dt.tz_convert` fixes the issue.

This is similar to but different from #25423 and #35997 as far as I understand.

See also [this StackOverflow discussion](https://stackoverflow.com/questions/66104233/pandas-read-parquet-pytz-exceptions-unknowntimezoneerror).

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

<details>
INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-73-generic
Version          : #82~18.04.1-Ubuntu SMP Fri Apr 16 15:10:02 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : 6.2.2
hypothesis       : None
sphinx           : 4.0.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 3.0.0
IPython          : 7.19.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None
</details>
