vfs: Fix deprecation warning in Python 3.10 (issue6520)

Differential Revision: https://phab.mercurial-scm.org/D10710

mercurial/vfs.py

@@ -307,7 +307,7 b' class abstractvfs(object):'
         # multiple instances puts us at risk of running out of file descriptors
         # only allow to use backgroundfilecloser when in main thread.
         if not isinstance(
-            threading.currentThread(),
+            threading.current_thread(),
             threading._MainThread,  # pytype: disable=module-attr
         ):
             yield
@@ -483,7 +483,7 b' class vfs(abstractvfs):'
             fp = checkambigatclosing(fp)
 
         if backgroundclose and isinstance(
-            threading.currentThread(),
+            threading.current_thread(),
             threading._MainThread,  # pytype: disable=module-attr
         ):
             if (