Rsdict

- Fix `get()`.
- Fix type checking.

- Fix `__setattr__()`.

- Key-restriction can be switched.
- Add initializing argument 'fixkey'.
- Type-restriction can be switched.
- Add initializing argument 'fixtype' (as replacement for 'strict').
- Add initializing argument 'cast'.
- Use GitHub Actions (Python package).
- Add support for `__ior__()`, `clear()`, `setdefault()`, `pop()`, `popitem()`, `del`.
- Improve test.

**Implemented enhancements:**
**Fixed bugs:**
**Closed issues:**
**Merged pull requests:**
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