Put back import of mpl_toolkits.mplot3d

Per recommendation of @dstrain115 in [review
comment](#432 (comment)),
putting back the import of `mplot3d`.

Author: mhucka

recirq/benchmarks/rabi_oscillations.py

@@ -18,6 +18,9 @@
 
 from matplotlib import pyplot as plt
 
+# this is for older systems with matplotlib <3.2 otherwise 3d projections fail
+from mpl_toolkits import mplot3d  # pylint: disable=unused-import
+
 import cirq