- update to 0.21.0:
* Improved the documentation, thanks to jonathanmach and Jakub
Wilk
* Fixed a thread-safety regression
* Improved the type annotations, thanks to David Pärsson
* Fixed singleton scope behavior with parent/child injectors,
thanks to David Pärsson
* Stopped using a deprecated test function, thanks to ljnsn

- Added support for PEP 604 union types (Python 3.10+), thanks to David Pärsson
- Dropped Python 3.6 support
- BuildRequires on the dataclasses module if using python 3.6
* Added support for overriding injectable parameters with positional
* Dropped Python 3.4 support
* Removed previously deprecated constructs: with_injector,
* Dependencies are no longer injected into Module.configure and
* Removed unofficial support for injecting into parent class