Python 3.10 support

[ptsavol]

Update spinedb_api to support Python 3.10

Re Spine-project/Spine-Toolbox#1700

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [ ] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass