Support Python 3.10

[hvy]

Motivation

Python 3.10 has been released https://www.python.org/downloads/release/python-3100/.

Since 3.10 users can be expected, we should make sure that Optuna works with this version.

Description

Add 3.10 to the unit test and optuna-example CI matrices. Make sure that all jobs complete successfully. Note that necessary Docker images and other tools may be missing, yet.

Additional context (optional)

See also the following issues/PRs from 3.9 for reference.

• Partial support of python 3.9 #1908
• Support Python 3.9 with integration modules and optional dependencies #2034

[HideakiImamura]

GitHub Actions allows to use Python 3.10. So, we would like to setup the CI.

[HideakiImamura]

actions/setup-python#249

[hugovk]

Python 3.10.0 is now available: actions/setup-python#249 (comment)

[toshihikoyanase]

We'd like to check this issue before releasing v3. Let me add the v3 label.

[semiexp]

I tested the compatibility with Python 3.10 in #3179, but pip install torch failed (torch does not support 3.10 yet): https://github.com/optuna/optuna/runs/4572899427?check_suite_focus=true

[hugovk]

Here's the torch issue: pytorch/pytorch#66424

[nzw0301]

FYI: PyTorch 1.11.0 has been released! This version starts to support Python 3.10.

[nzw0301]

Let me summarise the current status of this issue (at least on my local machine):

• I locally confirmed that the test of allennlp 2.9. 2 with torch 1.11.0 was successful without any modification; I could not reproduce CI failed due to the error in AllenNLP integration #3366.
• I locally confirmed that PyTorch Lightning 1.6.0 with torch 1.11.0 was successful if we modified the definition of pytorch-lightning callback as in this dirty PR.

Hence I think we can resolve this issue in the following steps:

1. Merge Partially support Python 3.10 #3353
2. Bump-up torch and related libraries except for PyTorchLightning and remove python 3.10 condition introduced by Partially support Python 3.10 #3353 (this is because PyTorch lightning's modification will not be as trivial as others.)
3. Update PyTorchLightningCallback and unpin its version.

[HideakiImamura]

We will work on PyTorchLightningCallback in the other issue, so we will remove it from the TODO of this issue.