towards python 2.x support for deep_mimic (still issue with Logger.print2)

examples/pybullet/gym/pybullet_envs/deep_mimic/learning/solvers/solver.py

@@ -1,4 +1,9 @@
-from abc import ABC, abstractmethod
+from abc import abstractmethod
+import sys, abc
+if sys.version_info >= (3, 4):
+    ABC = abc.ABC
+else:
+    ABC = abc.ABCMeta('ABC', (), {})
 
 class Solver(ABC):
     def __init__(self, vars):
@@ -7,4 +12,4 @@ class Solver(ABC):
 
     @abstractmethod
     def update(self, grads):
-        pass
+        pass