Mutwo.common

Added
- `common_generators.DynamicChoice`

Dropped
- Python 3.9 support

Added
- explicit exception classes in `mutwo.common_utilities`

Changed
- raised exceptions to custom exception classes
- package name from `mutwo.ext-common-generators` to `mutwo.common`

Added
- `Backtracking` and `IndexBasedBacktracking`

Added
- property `terminal_tuple` and `non_terminal_tuple` to `ContextFreeGrammar`