* Add type_caster<std::monostate> for std::variant
Add type_caster<std::monostate>, allowing std::variant<std::monostate, ...>
* Add variant<std::monostate, ...> test methods
* Add std::monostate tests
* Update test_stl.py
Remove erroneous extra tests
* [pre-commit.ci] auto fixes from pre-commit.com hooks
for more information, see https://pre-commit.ci
* Update test fn name
* And update the doc() test
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
