unpack: add can_unpack routine

capa/unpack/__init__.py

@@ -1,3 +1,4 @@
+import sys
 import logging
 
 try:
@@ -13,6 +14,11 @@ class NotPackedError(ValueError):
         super(NotPackedError, self).__init__("not packed")
 
 
+def can_unpack():
+    # the unpacking backend is based on Speakeasy, which supports python 3.6+
+    return sys.version_info >= (3, 6)
+
+
 @lru_cache
 def get_unpackers():
     # break import loop