fixes #7: console_scripts file names now have the major python version appended for python3

[hrudham]

See issue #7

Comment within the code explaining the change:

Some python debian packages have separate python2 and python3 Debian packages released. If a package has a console_scripts entry, and the python2 version of a debian package is already installed, when we attempt to install a python3 package created by wheel2deb, it will fail because apt will not let it overwrite an existing file. An example of this is the pyjwt package: for the officially released Debian packages, this creates /usr/bin/pyjwt (apt install python-jwt) and /usr/bin/pyjwt3 (apt install python3-jwt). The code below follows a similar pattern, by appending the python major version to console_scripts entries for versions three and above.