BUG: Reading from parquet throws UnknownTimeZoneError using timezone-aware date in index
- [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.

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

df = pd.DataFrame([[datetime.now(timezone.utc)]], columns=['date']).set_index('date')
df.to_parquet('out.parquet')
pd.read_parquet('out.parquet')
```

#### Problem description
The bug above happens with pandas 1.1.1 and pyarrow 1.0.1.
The timezone-aware date in the index should survive the parquet round trip.
If `date` is not index, or when I add parameter `ignore_metadata=True` to the `pyarrow.Table.to_pandas()` it works (but `date` won't be an index automatically)

#### Expected Output
A correct DataFrame

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

<details>
INSTALLED VERSIONS
------------------
commit           : f2ca0a2665b2d169c97de87b8e778dbed86aea07
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.104-microsoft-standard
Version          : #1 SMP Wed Feb 19 06:37:35 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.1
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.2
setuptools       : 49.6.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 1.0.1
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
