diff -r 142a76127e3a -r 2b76255a4f74 mercurial/pycompat.py
--- a/mercurial/pycompat.py	Thu Aug 19 18:04:14 2021 +0200
+++ b/mercurial/pycompat.py	Thu Aug 19 17:46:46 2021 +0200
@@ -40,6 +40,8 @@
     def future_set_exception_info(f, exc_info):
         f.set_exception_info(*exc_info)
 
+    # this is close enough for our usage
+    FileNotFoundError = OSError
 
 else:
     import concurrent.futures as futures
@@ -53,6 +55,8 @@
     def future_set_exception_info(f, exc_info):
         f.set_exception(exc_info[0])
 
+    FileNotFoundError = __builtins__['FileNotFoundError']
+
 
 def identity(a):
     return a