adds code to require a envvar to be able to use billiard

Author: FredworkLemmas

memory_profiler.py

@@ -22,7 +22,9 @@
 
 # TODO: provide alternative when multiprocessing is not available
 try:
-    from billiard import Process, Pipe
+    if "MEMPROF_PREFERS_BILLIARD" in os.environ and \
+            os.environ["MEMPROF_PREFERS_BILLIARD"] in ["True", "true", "TRUE", "1"]:
+        from billiard import Process, Pipe
 except ImportError:
     try:
         from multiprocessing import Process, Pipe