[BUG] mypy: Found 126 errors in 26 files (checked 44 source files) 

**Describe the bug**
Mypy found 126 errors in 26 files (checked 44 source files)

**To Reproduce**
Steps to reproduce the behavior: run the following command
```bash
mypy obsei/ --ignore-missing-imports
```
**Expected behavior**
No mypy errors should be there.

