Fix crash due to runtime import of typing_extensions

typing_extensions doesn't presently need to be a runtime dependency.

Author: gschaffner

trio/_unix_pipes.py

@@ -2,13 +2,16 @@
 
 import os
 import errno
-from typing_extensions import Final as FinalType
+from typing import TYPE_CHECKING
 
 from ._abc import Stream
 from ._util import ConflictDetector, Final
 
 import trio
 
+if TYPE_CHECKING:
+    from typing_extensions import Final as FinalType
+
 if os.name != "posix":
     # We raise an error here rather than gating the import in lowlevel.py
     # in order to keep jedi static analysis happy.