TypeAlias unsupported for 3.9

[ThomSerg]

#775 introduced type hints for solve and solveAll. One thing I overlooked when reviewing the PR is that TypeAlias is only supported from 3.10 onwards. Since in the future we want to add even more type hints, the use of TypeAlias is probably here to stay (allows for less verbose typehints). This would require us to drop support for 3.9.

[hbierlee]

Since 3.9 is end-of-life anyway (https://devguide.python.org/versions/), I think we should adopt the reasonable policy that only supported Python versions are supported by cpmpy. We once discussed this, not sure what argument against is (slightly less flexible?). Unless we have specific users we cater for that use end-of-life, perhaps 10.0.0 is a good point to adopt this policy.