Injector

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- 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

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- Added support for PEP 604 union types (Python 3.10+), thanks to David Pärsson
- Fixed building with pypandoc 1.8+, thanks to Søren Fuglede Jørgensen

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- Fixed handling of Union combined with Annotated, thanks to Tobias Nilsson
- Fixed AssitedBuilder/child Injector interaction, thanks to Erik Cederberg
- Made get_bindings() and injections work even if a injectee's return type
annotation is a forward reference that can't be resolved

Backwards incompatible:

- Dropped Python 3.6 support

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- Added the license to the source distribution, thanks to Joshua Adelman
- Added Python 3.9 and 3.10 support, this includes fixing Python 3.10 compatibility, thanks to Torge Matthies
- Improved the documentation, thanks to Takahiro Kojima
- Improved the source distribution so that it can be used to build and install wheels, thanks to Janusz Skonieczny
- Added requirements files for easier development, thanks to Greg Eremeev

Backwards incompatible:

- Removed Python 3.5 support

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- Fixed a bug where only one of multiple NoInject annotations was interpreted

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- Fixed Python 3.5.3 compatibility