Update checkpython.py to reflect py3.6 requirement

Author: jindrahelcl

neuralmonkey/checkpython.py

@@ -1,7 +1,7 @@
 import sys
 
-if sys.version_info[0] < 3 or sys.version_info[1] < 5:
+if sys.version_info[0] < 3 or sys.version_info[1] < 6:
     print("Error:", file=sys.stderr)
-    print("Neural Monkey must use Python >= 3.5", file=sys.stderr)
+    print("Neural Monkey must use Python >= 3.6", file=sys.stderr)
     print("Your Python is", sys.version, sys.executable, file=sys.stderr)
     sys.exit(1)