BUG: Unusual behaviour in to_parquet re-load file
- [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 pandas as pd
print(pd.__version__)

df = pd.DataFrame({'col1': [1,2,3,4,5], 'col2': ['s','p','d','q','r']})
# check data types
print(df.dtypes)  # <-- col1 dtype: int64

# convert col1 data type from int64 to object
df = df.astype({'col1': 'object'})
print(df.dtypes) # <-- col1 dtype: object

# write it to a parquet file
df.to_parquet('./test.parquet', compression='gzip')
re-load the dataset
df1 = pd.read_parquet('./test.parquet')
print(df1.dtypes) # <-- col1 dtype: int64 

```

#### Problem description

In the above code, col1 in df is converted to object before writing it to parquet file. Surprisingly, it is converted to integer after re-load. Is that an expected behaviour? I think it should preserve the data types of each column.

I have seen same behaviour for float values as well.

#### Expected Output
col1     object
col2    object
dtype: object

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

[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]

</details>

INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.7.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.3.0
Version          : Darwin Kernel Version 20.3.0: Thu Jan 21 00:07:06 PST 2021; root:xnu-7195.81.3~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.2.3
numpy            : 1.20.1
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 49.6.0.post20210108
Cython           : 0.29.22
pytest           : 6.2.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.21.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.52.0

