fix backgrounds and remove CP6 compatibility

[FoamyGuy]

resolves #28

There were a few options to resolve this:

1. remove with context and add a exception for pylint
2. remove the CP6 compatibility version now that 7 has a stable release. Starting with CP7 the filename can be passed directly to OnDiskBitmap which avoids the library needing to open or managed the open file at all. Instead we just pass the string filepath to OnDiskBitmap and it handles the file internally.

I went with option 2 since we do have a 7 stable release now and since doing it this way doesn't require the pylint exception.

Tested successfully on PyPortal:

Adafruit CircuitPython 7.1.0-beta.0 on 2021-11-12; Adafruit PyPortal with samd51j20
Board ID:pyportal

[FoamyGuy]

Here are some photos that illustrate this issue and fix.

Current version of library on the home screen of cyoa example:

Fixed version from this PR same page:

[tannewt]

Thank you!

[tannewt]

FYI I added today's bundle as the last supported 6.x bundle: https://learn.adafruit.com/welcome-to-circuitpython/frequently-asked-questions