generator: Don't force python2

Python2 is being phased out of the default python interpreter.  Arch
Linux has moved some time ago and upcoming Debian realeases will follow.

My distro, Arch, doesn't have a python2-protobuf version, but does have
a python3 version.  With a python 2 & 3 compatible generator the exact
interpreter can be ignored and can use the system default as most users
expect.

Update issue #155
1 file changed