Rewrite SessionBase properties to use @property

[adamchainz]

I have made things!

Follow-up to observation in: #1128 (comment)

Rewrite these properties to use @property.

Tested by adding the below at the bottom of django-stubs/contrib/sessions/backends/base.pyi:

reveal_type(SessionBase().session_key)
reveal_type(SessionBase()._session_key)
SessionBase()._session_key = 1

Running Mypy without the fix:

$ mypy django-stubs/contrib/sessions/backends/base.pyi
django-stubs/contrib/sessions/backends/base.pyi:45: note: Revealed type is "Any"
django-stubs/contrib/sessions/backends/base.pyi:46: note: Revealed type is "Any"
Success: no issues found in 1 source file

...and after:

$ mypy django-stubs/contrib/sessions/backends/base.pyi
django-stubs/contrib/sessions/backends/base.pyi:49: note: Revealed type is "builtins.str"
django-stubs/contrib/sessions/backends/base.pyi:50: note: Revealed type is "builtins.str"
django-stubs/contrib/sessions/backends/base.pyi:51: error: Incompatible types in assignment (expression has type "int", variable has type "str")
Found 1 error in 1 file (checked 1 source file)

The correct types are revealed, and the assignment is detected as incorrect.

Related issues

n/a