Merge pull request optuna#1561 from hvy/bp-1551

[Backport] Add `packaging` in install_requires.

Merge pull request optuna#1476 from toshihikoyanase/bump-to-v2.0.0rc0

Bump up version to `2.0.0rc0`.

Merge pull request optuna#1313 from toshihikoyanase/release-v1.5.0

Apply optuna#1291 and optuna#1306 to the `release-v1.5.0`.

Merge pull request optuna#1227 from toshihikoyanase/bump-to-v1.4.0

Bump up version number to 1.4.0.

Merge pull request optuna#1073 from hvy/bump-to-v1.3.0

Bump up version number to 1.3.0.

Merge pull request optuna#981 from toshihikoyanase/bump-to-v1.2.0

Bump up version number to 1.2.0.

Merge pull request optuna#898 from sile/bump-to-v1.1.0

Bump up version number to 1.1.0.

Merge pull request optuna#831 from hvy/bump-to-v1.0.0

Bump up version number to 1.0.0

Merge pull request optuna#709 from optuna/bump-to-v0.19.0

Bump up version number to 0.19.0.

Merge pull request optuna#680 from pfnet/bump-to-v0.18.1

Bump up version number to 0.18.1.