The Pico does not have specific pins labeled on the board as the default I2C, SPI, or UART pins. See the Pinouts page for more details.
If you are using CircuitPython 6.2.0-beta.1 or earlier, avoid using this pin in CircuitPython. Pin GP15 is used briefly by the internal USB peripheral on startup, and was not reset to the proper value after use. The "Absolute Newest builds" fix this problem. Please see this issue comment for more details.
pulseio is not yet supported. Please subscribe to this CircuitPython issue to be notified when it's been added.