fix bug in _dummy_thread.py that was introduced with CPython 3.7.4

Author: timfel

graalpython/lib-python/3/_dummy_thread.py

@@ -169,6 +169,8 @@ def acquire(self, waitflag=None, timeout=-1):
             self._levels += 1
         return locked
 
+    __enter__ = acquire
+
     def release(self):
         """Release needs to be called once for every call to acquire().
         """