fix version overriding arg

[FoamyGuy]

resolves #218

If --cpy-version CLI argument is passed by user, then pass it's value to the Backend constructor and store it in a local variable to be returned if it exists from get_circuitpython_version()

I was able to successfully test this version with these commands:

circup --path fake_project/ --cpy-version 8.0.0 --board-id hello install adafruit_display_text
circup --path fake_project/ --cpy-version 9.0.0 --board-id hello install adafruit_display_text

[tannewt]

Thank you!