feat: Add variance handling for the stubs generator.

[Masara]

Is your feature request related to a problem?

Currently code with variances is not handled during the stubs generation. If possible, this feature should be added.

Desired solution

In Python variance is inferred, therefor, if possible with Mypy, it should be inferred and added to the stubs generator.

Additional Context (optional)

• https://dsl.safeds.com/en/latest/language/common/variance/
• https://github.com/Safe-DS/DSL/blob/4c61b8c18a50fc89f193ebbbaa66ef1743db9ee0/tests/resources/grammar/declarations/classes/good-covariant%20type%20parameter.sdstest
• https://github.com/Safe-DS/DSL/blob/4c61b8c18a50fc89f193ebbbaa66ef1743db9ee0/tests/resources/grammar/declarations/classes/good-contravariant%20type%20parameter.sdstest
• https://peps.python.org/pep-0695/#summary

[Masara]

@lars-reimann With issue #36 we are able to create stubs like these:

# Python Code
_T_co = TypeVar("_T_co", covariant=True, bound=str)
_T_con = TypeVar("_T_con", contravariant=True, bound=int)

class ClassA(Generic[_T_co, _T_con]):
        ...

// Safe-DS stubs
class ClassA<out TCo, in TCon> where {
    TCo sub String,
    TCon super Int
}

Therefore, this Issue is also already finished, right? Or am I mixing variance and constrains up?

[lars-reimann]

You can naturally handle constraints and variance in one PR, but they are different concepts:

Variance:

• Python covariance=True -> Safe-DS out
• Python contravariance=True -> Safe-DS in
• Python neither -> Safe-DS no modifier

Bounds/constraints:

• Python bound=... -> Safe-DS where block with type parameter constraints

[Masara]

Since I handled both concepts already in issue #36 I will close this issue.