#7 UsingExtension flag to be externally accessible in Python 3

[Joe Walker]

Add application access to the "usingExtension" flag so that applications using the crcmod package can determine if the c extension is being used or not. (Specifically for Python 3)

EX:
Adding the following line to /python3/crcmod/init.py

from crcmod.crcmod import _usingExtension