fix(types): tighten type for venv_backend (#967)

Author: henryiii

nox/_decorators.py

@@ -71,7 +71,7 @@ def __init__(
         python: _typing.Python = None,
         reuse_venv: bool | None = None,
         name: str | None = None,
-        venv_backend: Any = None,
+        venv_backend: str | None = None,
         venv_params: Sequence[str] = (),
         should_warn: Mapping[str, Any] | None = None,
         tags: Sequence[str] | None = None,

nox/registry.py

@@ -49,7 +49,7 @@ def session_decorator(
     py: Python | None = ...,
     reuse_venv: bool | None = ...,
     name: str | None = ...,
-    venv_backend: Any | None = ...,
+    venv_backend: str | None = ...,
     venv_params: Sequence[str] = ...,
     tags: Sequence[str] | None = ...,
     *,
@@ -65,7 +65,7 @@ def session_decorator(
     py: Python | None = None,
     reuse_venv: bool | None = None,
     name: str | None = None,
-    venv_backend: Any | None = None,
+    venv_backend: str | None = None,
     venv_params: Sequence[str] = (),
     tags: Sequence[str] | None = None,
     *,