groupby.size may return DataFrame in as_index=False

[MarcoGorelli]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

1. Provide a minimal runnable pandas example that is not properly checked by the stubs.
2. Indicate which type checker you are using (mypy or pyright).
3. Show the error message received from that type checker while checking your example.

t.py

import pandas as pd

df = pd.DataFrame({'a': [1,2,3]})
reveal_type(df.groupby('a', as_index=False).size())

$ mypy t.py
t.py:4: note: Revealed type is "pandas.core.series.Series[builtins.int]"

However, it is in fact a DataFrame

Please complete the following information:

• OS: [e.g. Windows, Linux, MacOS] linux
• OS Version [e.g. 22] Linux DESKTOP-U8OKFP3 5.10.102.1-microsoft-standard-WSL2 Tasks to complete before making this repo public #1 SMP Wed Mar 2 00:30:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
• python version 3.8
• version of type checker
• version of installed pandas-stubs

$ pip freeze | grep pandas
pandas==2.0.0
pandas-flavor==0.5.0
pandas-stubs==2.0.0.230412

Additional context
Add any other context about the problem here.

[Dr-Irv]

Related to #203

[loicdiridollou]

I think we can close this one @Dr-Irv as this was taken care of in #1025 and #1045.