Issue #13809: Make bz2 module work with threads disabled.

Original patch by Amaury Forgeot d'Arc.
diff --git a/Lib/bz2.py b/Lib/bz2.py
index 5c59a9e..36e5558 100644
--- a/Lib/bz2.py
+++ b/Lib/bz2.py
@@ -10,9 +10,13 @@
 __author__ = "Nadeem Vawda <nadeem.vawda@gmail.com>"
 
 import io
-import threading
 import warnings
 
+try:
+    from threading import RLock
+except ImportError:
+    from dummy_threading import RLock
+
 from _bz2 import BZ2Compressor, BZ2Decompressor
 
 
@@ -53,7 +57,7 @@
         """
         # This lock must be recursive, so that BufferedIOBase's
         # readline(), readlines() and writelines() don't deadlock.
-        self._lock = threading.RLock()
+        self._lock = RLock()
         self._fp = None
         self._closefp = False
         self._mode = _MODE_CLOSED