Works around issue3863: freebsd4/5/6 and os2emx are known to have OS bugs when
calling fork() from a child thread.  This disables that unit test (with a note
printed to stderr) on those platforms.

A caveat about buggy platforms is added to the os.fork documentation.
diff --git a/Misc/NEWS b/Misc/NEWS
index 1b08d76..6e6c58d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -37,6 +37,9 @@
 
 - Issue #3879: A regression in urllib.getproxies_enviroment was fixed.
 
+- Issue #3863: Disabled a unit test of fork being called from a thread
+  when running on platforms known to exhibit OS bugs when attempting that.
+
 Build
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