Merge pull request #2537 from gschaffner/missing-typing_extensions-crash

Fix crash due to runtime import of `typing_extensions`

Author: pquentin

trio/_core/_run.py

@@ -23,9 +23,6 @@
 from sniffio import current_async_library_cvar
 from sortedcontainers import SortedDict
 
-# An unfortunate name collision here with trio._util.Final
-from typing_extensions import Final as FinalT
-
 from .. import _core
 from .._util import Final, NoPublicConstructor, coroutine_or_error
 from ._asyncgens import AsyncGenerators
@@ -47,6 +44,10 @@
 if sys.version_info < (3, 11):
     from exceptiongroup import BaseExceptionGroup
 
+if TYPE_CHECKING:
+    # An unfortunate name collision here with trio._util.Final
+    from typing_extensions import Final as FinalT
+
 DEADLINE_HEAP_MIN_PRUNE_THRESHOLD: FinalT = 1000
 
 _NO_SEND: FinalT = object()