ENH: df.to_parquet() should return bytes
#### Is your feature request related to a problem?

I find it useful to write a parquet to a `bytes` object for some unit tests. The code that I currently use to do this is quite verbose.

To provide some background, `df.to_csv()` (w/o args) just works. It returns a `str` object as is expected. In the same vein, **`df.to_parquet()` (w/o args) should return a `bytes` object.**

More precisely, the current behavior is:
```python
>>> df = pd.DataFrame()

>>> type(df.to_csv())  # This works
<class 'str'>

>>> df.to_parquet() # This should be made to work
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: to_parquet() missing 1 required positional argument: 'path'
```

#### Describe the solution you'd like

The requested behavior is:
```python
>>> df = pd.DataFrame()

>>> type(df.to_parquet())
<class 'bytes'>
```

Other uses of `df.to_parquet` should obviously remain unaffected.

#### API breaking implications

It won't break the documented API.

#### Describe alternatives you've considered

I currently use this verbose code to get what I want:
```python
import io

import pandas as pd

df = pd.DataFrame()
pq_file = io.BytesIO()
df.to_parquet(pq_file)
pq_bytes = pq_file.getvalue()
```
This workaround is too effortful.