TI 83 keyboard to USB/raspbian?
Those keyboards are not capacitive as anyone would call them. It is a keyboard matrix with conductive carbon on the rubber pushbutton, called "Silicone Rubber Keypads".

The two ways you can get this going is either wire into the existing pcb matrix, or use a custom pcb. Both would normally go to either a USB keyboard you have hacked up, taking consideration for the existing matrix's pinout/connections, or make your own, with like a usb enable arduino or similar. A USB keyboard matrix is not a trivial project, but easy enough with the number of projects on the web. Google v-usb keyboard matrix.

For example, someone hacked a Game Boy into a TI-83 case, and reused the buttons: