Fix: move typing_extensions import under TYPE_CHECKING

[vgvoleg]

Summary

Fixes #37.

cursors.py imported Self from typing_extensions at module level, but typing-extensions was not declared as an explicit dependency. This broke with Poetry 2.x on Python 3.12+, where the package is not installed (its marker is computed as python_version < "3.11" from transitive deps).

Fix: since from __future__ import annotations is already present, Self is only used in type annotations that are never evaluated at runtime. Moving the import under if TYPE_CHECKING: eliminates the runtime dependency entirely — no changes to pyproject.toml needed.

Changes

• ydb_dbapi/cursors.py: moved from typing_extensions import Self under if TYPE_CHECKING:

Test plan

• python -c "import ydb_dbapi" succeeds on Python 3.12+ without typing-extensions installed
• ruff check passes
• mypy passes