Apparently in the flexmock universe, sys.stdout.write() takes two
arguments rather than one. As of late July 2021, upstream hasn't said
a word of comment on this; see
https://github.com/flexmock/flexmock/issues/37

--- a/tests/unit/commands/test_borgmatic.py
+++ b/tests/unit/commands/test_borgmatic.py
@@ -448,7 +448,10 @@
         ['baz']
     )
     stdout = flexmock()
-    stdout.should_receive('write').with_args('["foo", "bar", "baz"]').once()
+    try:
+        stdout.should_receive('write').with_args('["foo", "bar", "baz"]').once()
+    except flexmock.MethodSignatureError:
+        stdout.should_receive('write').with_args('["foo", "bar", "baz"]', None).once()
     flexmock(module.sys).stdout = stdout
     arguments = {}