Multiple processes (e.g. -j2) causes some missing stdout

[goodoldneon]

Environment

future:   0.18.2
python:   3.8.2
OS:       MacOS 10.15.4

Issue

When I run future with multiple processes, the "RefactoringTool: Refactored" stdout is missing.

For example, I created the following file:

# foo.py
print 1

When running future with multiple processes, I get the following output:

$ futurize -j4 foo.py
RefactoringTool: Skipping optional fixer: idioms
RefactoringTool: Skipping optional fixer: ws_comma
--- foo.py	(original)
+++ foo.py	(refactored)
@@ -1 +1,2 @@
-print 1
+from __future__ import print_function
+print(1)
RefactoringTool: No files need to be modified.

When running future in one process, I get the following output:

$ futurize foo.py
RefactoringTool: Skipping optional fixer: idioms
RefactoringTool: Skipping optional fixer: ws_comma
RefactoringTool: Refactored foo.py
--- foo.py	(original)
+++ foo.py	(refactored)
@@ -1 +1,2 @@
-print 1
+from __future__ import print_function
+print(1)
RefactoringTool: Files that need to be modified:
RefactoringTool: foo.py

It seems like futurize -j4 is able to correctly analyze the file, but also doesn't recognize that the file exists.