Update linecache.pyi (#7623)

The `mtime` in linecache entries can be set to `None` for files loaded via a `__loader__` (cf: https://github.com/python/cpython/blob/3.10/Lib/linecache.py#L70)

Author: till-varoquaux

stdlib/linecache.pyi

@@ -7,7 +7,7 @@ else:
     __all__ = ["getline", "clearcache", "checkcache"]
 
 _ModuleGlobals = dict[str, Any]
-_ModuleMetadata = tuple[int, float, list[str], str]
+_ModuleMetadata = tuple[int, float | None, list[str], str]
 
 class _SourceLoader(Protocol):
     def __call__(self) -> str | None: ...